Enable the MergeFunc pass

Fixes #9536 

---

From https://github.com/rust-lang/rust/issues/9536#issuecomment-45495670:

I've built rustc with the aforementioned fix, once with MergeFunc being run early (that's what the patch for clang that comes  with LLVM does), and once with MergeFunc being run late (using `-C passes=mergefunc`). Here are some time/code size measurements I made with them:

Build command: `rustc -O -o /dev/null --emit asm .../lib.rs`

CPU (user) times, best of three runs:

 Crate       | No MergeFunc | Early MergeFunc | Late MergeFunc
-------------|--------------|-----------------|---------------
 core        |    5.380s    |    5.476s       |    5.364s
 collections |    1.884s    |    1.856s       |    1.892s
 native      |    7.200s    |    7.356s       |    7.108s
 rustc       | 3m23.584s    | 3m28.120s       | 3m21.820s
 std         |   13.888s    |   13.976s       |   13.848s
 syntax      |   48.992s    |   47.752s       |   48.372s

Sizes:

Crate                                    | No MergeFunc |   Early MergeFunc   |    Late MergeFunc
-----------------------------------------|-------------:|--------------------:|-------------------:
lib                                      |  237037581   |  236005998 (-0.44%) |  234708744 (-0.98%)
libarena-063bff73-0.11.0-pre.so          |      60398   |      60393 (-0.01%) |      60394 (-0.01%)
libcollections-d412c0c4-0.11.0-pre.so    |     971566   |     971772 (+0.02%) |     971691 (+0.01%)
libdebug-1e940314-0.11.0-pre.so          |     181352   |     181514 (+0.09%) |     181363 (+0.01%)
libflate-92afea7e-0.11.0-pre.so          |     137837   |     137869 (+0.02%) |     137837 (+0.00%)
libfmt_macros-5125f3bd-0.11.0-pre.so     |     132733   |     134598 (+1.41%) |     132465 (-0.20%)
libgetopts-c94737d1-0.11.0-pre.so        |     158851   |     157427 (-0.90%) |     158272 (-0.36%)
libgraphviz-7b3cf89d-0.11.0-pre.so       |      53337   |      53178 (-0.30%) |      53337 (+0.00%)
liblog-cd053230-0.11.0-pre.so            |      85993   |      86017 (+0.03%) |      85780 (-0.25%)
libnative-1fb5e2c0-0.11.0-pre.so         |     635785   |     639352 (+0.56%) |     621184 (-2.30%)
libregex-77385931-0.11.0-pre.so          |     450538   |     450741 (+0.05%) |     449504 (-0.23%)
librustc-d252d482-0.11.0-pre.so          |   51583741   |   51227703 (-0.69%) |   50930784 (-1.27%)
librustdoc-6ecbf63e-0.11.0-pre.so        |    4557104   |    4501896 (-1.21%) |    4394409 (-3.57%)
libserialize-0352aab7-0.11.0-pre.so      |    1126096   |    1115503 (-0.94%) |    1101734 (-2.16%)
libstd-59beb4f7-0.11.0-pre.so            |    4499529   |    4488879 (-0.24%) |    4477710 (-0.48%)
libsync-305341d2-0.11.0-pre.so           |     306767   |     312211 (+1.77%) |     304086 (-0.87%)
libsyntax-555559ea-0.11.0-pre.so         |    6699751   |    6632291 (-1.01%) |    6596232 (-1.55%)
libterm-4e4945a5-0.11.0-pre.so           |     389390   |     392689 (+0.85%) |     385525 (-0.99%)
libtest-a79f950d-0.11.0-pre.so           |     740161   |     730673 (-1.28%) |     734534 (-0.76%)
libtime-4bb3739b-0.11.0-pre.so           |     131518   |     132830 (+1.00%) |     131514 (-0.00%)
rustlib                                  |  164131038   |  163594366 (-0.33%) |  162796293 (-0.81%)
x86_64-unknown-linux-gnu                 |  164119867   |  163583195 (-0.33%) |  162785122 (-0.81%)
lib                                      |  164115771   |  163579099 (-0.33%) |  162781026 (-0.81%)
liballoc-1085c790-0.11.0-pre.rlib        |    1094410   |    1094444 (+0.00%) |    1094438 (+0.00%)
libarena-063bff73-0.11.0-pre.rlib        |     312324   |     312152 (-0.06%) |     312210 (-0.04%)
libarena-063bff73-0.11.0-pre.so          |      60394   |      60394 (+0.00%) |      60394 (+0.00%)
libcollections-d412c0c4-0.11.0-pre.rlib  |    7048646   |    7049094 (+0.01%) |    7048856 (+0.00%)
libcollections-d412c0c4-0.11.0-pre.so    |     971575   |     971771 (+0.02%) |     971681 (+0.01%)
libcompiler-rt.a                         |     573802   |     573802 (+0.00%) |     573802 (+0.00%)
libcore-c5ed6fb4-0.11.0-pre.rlib         |   24204746   |   24209820 (+0.02%) |   24187602 (-0.07%)
libdebug-1e940314-0.11.0-pre.rlib        |     876616   |     878488 (+0.21%) |     876746 (+0.01%)
libdebug-1e940314-0.11.0-pre.so          |     181352   |     181509 (+0.09%) |     181353 (+0.00%)
libflate-92afea7e-0.11.0-pre.rlib        |     175062   |     175074 (+0.01%) |     175082 (+0.01%)
libflate-92afea7e-0.11.0-pre.so          |     137837   |     137869 (+0.02%) |     137837 (+0.00%)
libfmt_macros-5125f3bd-0.11.0-pre.so     |     132724   |     134599 (+1.41%) |     132469 (-0.19%)
libfourcc-cc0e8bf1-0.11.0-pre.so         |     125828   |     126084 (+0.20%) |     125827 (-0.00%)
libgetopts-c94737d1-0.11.0-pre.rlib      |     864664   |     853040 (-1.34%) |     862548 (-0.24%)
libgetopts-c94737d1-0.11.0-pre.so        |     158855   |     157425 (-0.90%) |     158275 (-0.37%)
libglob-eafe1d22-0.11.0-pre.rlib         |     951370   |     944674 (-0.70%) |     946734 (-0.49%)
libglob-eafe1d22-0.11.0-pre.so           |     159130   |     157385 (-1.10%) |     156791 (-1.47%)
libgraphviz-7b3cf89d-0.11.0-pre.rlib     |     269600   |     269062 (-0.20%) |     269560 (-0.01%)
libgraphviz-7b3cf89d-0.11.0-pre.so       |      53334   |      53176 (-0.30%) |      53337 (+0.01%)
libgreen-ca0d0b80-0.11.0-pre.rlib        |    1374120   |    1389510 (+1.12%) |    1361696 (-0.90%)
libgreen-ca0d0b80-0.11.0-pre.so          |     372435   |     377929 (+1.48%) |     370991 (-0.39%)
libhexfloat-3b978f48-0.11.0-pre.so       |     131926   |     132166 (+0.18%) |     131935 (+0.01%)
liblibc-4f9a876d-0.11.0-pre.rlib         |     617472   |     617472 (+0.00%) |     617472 (+0.00%)
liblog-cd053230-0.11.0-pre.rlib          |     371190   |     371048 (-0.04%) |     370836 (-0.10%)
liblog-cd053230-0.11.0-pre.so            |      85996   |      86020 (+0.03%) |      85781 (-0.25%)
libmorestack.a                           |       1388   |       1388 (+0.00%) |       1388 (+0.00%)
libnative-1fb5e2c0-0.11.0-pre.rlib       |    2233070   |    2264296 (+1.40%) |    2194920 (-1.71%)
libnative-1fb5e2c0-0.11.0-pre.so         |     635787   |     639341 (+0.56%) |     621184 (-2.30%)
libnum-ebe12db7-0.11.0-pre.rlib          |    2672318   |    2675292 (+0.11%) |    2669370 (-0.11%)
libnum-ebe12db7-0.11.0-pre.so            |     398924   |     399357 (+0.11%) |     395821 (-0.78%)
librand-2ea8f361-0.11.0-pre.rlib         |    1691108   |    1691696 (+0.03%) |    1690264 (-0.05%)
libregex-77385931-0.11.0-pre.rlib        |    2007348   |    2006050 (-0.06%) |    2003804 (-0.18%)
libregex-77385931-0.11.0-pre.so          |     450520   |     450790 (+0.06%) |     449535 (-0.22%)
libregex_macros-a2216dec-0.11.0-pre.so   |     597208   |     569004 (-4.72%) |     568800 (-4.76%)
librlibc-d1ece24e-0.11.0-pre.rlib        |      12394   |      12394 (+0.00%) |      12394 (+0.00%)
librustc-d252d482-0.11.0-pre.so          |   51582383   |   51230320 (-0.68%) |   50930784 (-1.26%)
librustdoc-6ecbf63e-0.11.0-pre.so        |    4557074   |    4501877 (-1.21%) |    4394506 (-3.57%)
librustuv-ede8cb89-0.11.0-pre.rlib       |    4774956   |    4791366 (+0.34%) |    4732386 (-0.89%)
librustuv-ede8cb89-0.11.0-pre.so         |    1401710   |    1400237 (-0.11%) |    1386869 (-1.06%)
libsemver-e49a2dee-0.11.0-pre.rlib       |     392704   |     392434 (-0.07%) |     392940 (+0.06%)
libsemver-e49a2dee-0.11.0-pre.so         |      71863   |      71847 (-0.02%) |      71860 (-0.00%)
libserialize-0352aab7-0.11.0-pre.rlib    |    8059698   |    8033972 (-0.32%) |    7989802 (-0.87%)
libserialize-0352aab7-0.11.0-pre.so      |    1126099   |    1115520 (-0.94%) |    1101721 (-2.16%)
libstd-59beb4f7-0.11.0-pre.rlib          |   18802728   |   18780212 (-0.12%) |   18743438 (-0.32%)
libstd-59beb4f7-0.11.0-pre.so            |    4499534   |    4488835 (-0.24%) |    4477677 (-0.49%)
libsync-305341d2-0.11.0-pre.rlib         |    1377062   |    1400190 (+1.68%) |    1369498 (-0.55%)
libsync-305341d2-0.11.0-pre.so           |     306762   |     312212 (+1.78%) |     304095 (-0.87%)
libsyntax-555559ea-0.11.0-pre.so         |    6703330   |    6632254 (-1.06%) |    6596295 (-1.60%)
libterm-4e4945a5-0.11.0-pre.rlib         |    1503928   |    1512648 (+0.58%) |    1495932 (-0.53%)
libterm-4e4945a5-0.11.0-pre.so           |     389380   |     392678 (+0.85%) |     385517 (-0.99%)
libtest-a79f950d-0.11.0-pre.rlib         |    3606962   |    3555410 (-1.43%) |    3589258 (-0.49%)
libtest-a79f950d-0.11.0-pre.so           |     740255   |     728398 (-1.60%) |     734521 (-0.77%)
libtime-4bb3739b-0.11.0-pre.rlib         |     847036   |     847178 (+0.02%) |     847194 (+0.02%)
libtime-4bb3739b-0.11.0-pre.so           |     131516   |     132829 (+1.00%) |     131518 (+0.00%)
liburl-b8b5640c-0.11.0-pre.rlib          |     647764   |     647558 (-0.03%) |     646896 (-0.13%)
liburl-b8b5640c-0.11.0-pre.so            |     146616   |     146621 (+0.00%) |     146531 (-0.06%)
libuuid-238d8f44-0.11.0-pre.rlib         |     359732   |     359682 (-0.01%) |     358936 (-0.22%)
libuuid-238d8f44-0.11.0-pre.so           |      77110   |      77110 (+0.00%) |      77023 (-0.11%)
total                                    |  237081476   |  236049893 (-0.44%) |  234752639 (-0.98%)


So running MergeFunc early like in the clang patch isn't nearly as good as running it late. I also tried to enable usage of global aliases instead of just thunks when merging functions, but that crashes.