In the fast path for decompressing literals, instead of checking
whether there's 16 bytes free and then checking right afterwards
(when having subtracted the literal size) that there are now 
5 bytes free, just check once for 21 bytes. This skips a compare
and a branch; although it is easily predictable, it is still
a few cycles on a fast path that we would like to get rid of.

Benchmarking this yields very confusing results. On open-source
GCC 4.8.1 on Haswell, we get exactly the expected results; the
benchmarks where we hit the fast path for literals (in particular
the two HTML benchmarks and the protobuf benchmark) give very nice
speedups, and the others are not really affected.

However, benchmarks with Google's GCC branch on other hardware
is much less clear. It seems that we have a weak loss in some cases
(and the win for the “typical” win cases are not nearly as clear),
but that it depends on microarchitecture and plain luck in how we run
the benchmark. Looking at the generated assembler, it seems that
the removal of the if causes other large-scale changes in how the
function is laid out, which makes it likely that this is just bad luck.

Thus, we should keep this change, even though its exact current impact is
unclear; it's a sensible change per se, and dropping it on the basis of
microoptimization for a given compiler (or even branch of a compiler)
would seem like a bad strategy in the long run.

Microbenchmark results (all in 64-bit, opt mode):

  Nehalem, Google GCC:

  Benchmark                Base (ns)  New (ns)                       Improvement
  ------------------------------------------------------------------------------
  BM_UFlat/0                   76747     75591  1.3GB/s  html           +1.5%
  BM_UFlat/1                  765756    757040  886.3MB/s  urls         +1.2%
  BM_UFlat/2                   10867     10893  10.9GB/s  jpg           -0.2%
  BM_UFlat/3                     124       131  1.4GB/s  jpg_200        -5.3%
  BM_UFlat/4                   31663     31596  2.8GB/s  pdf            +0.2%
  BM_UFlat/5                  314162    308176  1.2GB/s  html4          +1.9%
  BM_UFlat/6                   29668     29746  790.6MB/s  cp           -0.3%
  BM_UFlat/7                   12958     13386  796.4MB/s  c            -3.2%
  BM_UFlat/8                    3596      3682  966.0MB/s  lsp          -2.3%
  BM_UFlat/9                 1019193   1033493  953.3MB/s  xls          -1.4%
  BM_UFlat/10                    239       247  775.3MB/s  xls_200      -3.2%
  BM_UFlat/11                 236411    240271  606.9MB/s  txt1         -1.6%
  BM_UFlat/12                 206639    209768  571.2MB/s  txt2         -1.5%
  BM_UFlat/13                 627803    635722  641.4MB/s  txt3         -1.2%
  BM_UFlat/14                 845932    857816  538.2MB/s  txt4         -1.4%
  BM_UFlat/15                 402107    391670  1.2GB/s  bin            +2.7%
  BM_UFlat/16                    283       279  683.6MB/s  bin_200      +1.4%
  BM_UFlat/17                  46070     46815  781.5MB/s  sum          -1.6%
  BM_UFlat/18                   5053      5163  782.0MB/s  man          -2.1%
  BM_UFlat/19                  79721     76581  1.4GB/s  pb             +4.1%
  BM_UFlat/20                 251158    252330  697.5MB/s  gaviota      -0.5%
  Sum of all benchmarks      4966150   4980396                          -0.3%


  Sandy Bridge, Google GCC:
  
  Benchmark                Base (ns)  New (ns)                       Improvement
  ------------------------------------------------------------------------------
  BM_UFlat/0                   42850     42182  2.3GB/s  html           +1.6%
  BM_UFlat/1                  525660    515816  1.3GB/s  urls           +1.9%
  BM_UFlat/2                    7173      7283  16.3GB/s  jpg           -1.5%
  BM_UFlat/3                      92        91  2.1GB/s  jpg_200        +1.1%
  BM_UFlat/4                   15147     14872  5.9GB/s  pdf            +1.8%
  BM_UFlat/5                  199936    192116  2.0GB/s  html4          +4.1%
  BM_UFlat/6                   12796     12443  1.8GB/s  cp             +2.8%
  BM_UFlat/7                    6588      6400  1.6GB/s  c              +2.9%
  BM_UFlat/8                    2010      1951  1.8GB/s  lsp            +3.0%
  BM_UFlat/9                  761124    763049  1.3GB/s  xls            -0.3%
  BM_UFlat/10                    186       189  1016.1MB/s  xls_200     -1.6%
  BM_UFlat/11                 159354    158460  918.6MB/s  txt1         +0.6%
  BM_UFlat/12                 139732    139950  856.1MB/s  txt2         -0.2%
  BM_UFlat/13                 429917    425027  961.7MB/s  txt3         +1.2%
  BM_UFlat/14                 585255    587324  785.8MB/s  txt4         -0.4%
  BM_UFlat/15                 276186    266173  1.8GB/s  bin            +3.8%
  BM_UFlat/16                    205       207  925.5MB/s  bin_200      -1.0%
  BM_UFlat/17                  24925     24935  1.4GB/s  sum            -0.0%
  BM_UFlat/18                   2632      2576  1.5GB/s  man            +2.2%
  BM_UFlat/19                  40546     39108  2.8GB/s  pb             +3.7%
  BM_UFlat/20                 175803    168209  1048.9MB/s  gaviota     +4.5%
  Sum of all benchmarks      3408117   3368361                          +1.2%


  Haswell, upstream GCC 4.8.1:

  Benchmark                Base (ns)  New (ns)                       Improvement
  ------------------------------------------------------------------------------
  BM_UFlat/0                   46308     40641  2.3GB/s  html          +13.9%
  BM_UFlat/1                  513385    514706  1.3GB/s  urls           -0.3%
  BM_UFlat/2                    6197      6151  19.2GB/s  jpg           +0.7%
  BM_UFlat/3                      61        61  3.0GB/s  jpg_200        +0.0%
  BM_UFlat/4                   13551     13429  6.5GB/s  pdf            +0.9%
  BM_UFlat/5                  198317    190243  2.0GB/s  html4          +4.2%
  BM_UFlat/6                   14768     12560  1.8GB/s  cp            +17.6%
  BM_UFlat/7                    6453      6447  1.6GB/s  c              +0.1%
  BM_UFlat/8                    1991      1980  1.8GB/s  lsp            +0.6%
  BM_UFlat/9                  766947    770424  1.2GB/s  xls            -0.5%
  BM_UFlat/10                    170       169  1.1GB/s  xls_200        +0.6%
  BM_UFlat/11                 164350    163554  888.7MB/s  txt1         +0.5%
  BM_UFlat/12                 145444    143830  832.1MB/s  txt2         +1.1%
  BM_UFlat/13                 437849    438413  929.2MB/s  txt3         -0.1%
  BM_UFlat/14                 603587    605309  759.8MB/s  txt4         -0.3%
  BM_UFlat/15                 249799    248067  1.9GB/s  bin            +0.7%
  BM_UFlat/16                    191       188  1011.4MB/s  bin_200     +1.6%
  BM_UFlat/17                  26064     24778  1.4GB/s  sum            +5.2%
  BM_UFlat/18                   2620      2601  1.5GB/s  man            +0.7%
  BM_UFlat/19                  44551     37373  3.0GB/s  pb            +19.2%
  BM_UFlat/20                 165408    164584  1.0GB/s  gaviota        +0.5%
  Sum of all benchmarks      3408011   3385508                          +0.7%