cmd/gc: elide write barrier for x = x[0:y] and x = append(x, ...)

Both of these forms can avoid writing to the base pointer in x
(in the slice, always, and in the append, most of the time).

For Go 1.5, will need to change the compilation of x = x[0:y]
to avoid writing to the base pointer, so that the elision is safe,
and will need to change the compilation of x = append(x, ...)
to write to the base pointer (through a barrier) only when
growing the underlying array, so that the general elision is safe.

For Go 1.4, elide the write barrier always, a change that should
have equivalent performance characteristics but is much
simpler and therefore safer.

benchmark                       old ns/op   new ns/op   delta
BenchmarkBinaryTree17           3910526122  3918802545  +0.21%
BenchmarkFannkuch11             3747650699  3732600693  -0.40%
BenchmarkFmtFprintfEmpty        106         98.7        -6.89%
BenchmarkFmtFprintfString       280         269         -3.93%
BenchmarkFmtFprintfInt          296         282         -4.73%
BenchmarkFmtFprintfIntInt       467         470         +0.64%
BenchmarkFmtFprintfPrefixedInt  418         398         -4.78%
BenchmarkFmtFprintfFloat        574         535         -6.79%
BenchmarkFmtManyArgs            1768        1818        +2.83%
BenchmarkGobDecode              14916799    14925182    +0.06%
BenchmarkGobEncode              14110076    13358298    -5.33%
BenchmarkGzip                   546609795   542630402   -0.73%
BenchmarkGunzip                 136270657   136496277   +0.17%
BenchmarkHTTPClientServer       126574      125245      -1.05%
BenchmarkJSONEncode             30006238    27862354    -7.14%
BenchmarkJSONDecode             106020889   102664600   -3.17%
BenchmarkMandelbrot200          5793550     5818320     +0.43%
BenchmarkGoParse                5437608     5463962     +0.48%
BenchmarkRegexpMatchEasy0_32    192         179         -6.77%
BenchmarkRegexpMatchEasy0_1K    462         460         -0.43%
BenchmarkRegexpMatchEasy1_32    168         153         -8.93%
BenchmarkRegexpMatchEasy1_1K    1420        1280        -9.86%
BenchmarkRegexpMatchMedium_32   338         286         -15.38%
BenchmarkRegexpMatchMedium_1K   107435      98027       -8.76%
BenchmarkRegexpMatchHard_32     5941        4846        -18.43%
BenchmarkRegexpMatchHard_1K     185965      153830      -17.28%
BenchmarkRevcomp                795497458   798447829   +0.37%
BenchmarkTemplate               132091559   134938425   +2.16%
BenchmarkTimeParse              604         608         +0.66%
BenchmarkTimeFormat             551         548         -0.54%

LGTM=r
R=r, dave
CC=golang-codereviews, iant, khr, rlh
https://codereview.appspot.com/159960043