cmd/7g: enable peephole optimizer

Based on cmd/9g/peep.go.

Go 1 benchmark comparison:
benchmark                          old ns/op       new ns/op       delta
BenchmarkBinaryTree17              24328574000     18351639000     -24.57%
BenchmarkFannkuch11                17029365000     10817758000     -36.48%
BenchmarkFmtFprintfEmpty           291             223             -23.37%
BenchmarkFmtFprintfString          1073            799             -25.54%
BenchmarkFmtFprintfInt             1024            778             -24.02%
BenchmarkFmtFprintfIntInt          1654            1277            -22.79%
BenchmarkFmtFprintfPrefixedInt     1360            1083            -20.37%
BenchmarkFmtFprintfFloat           2272            1415            -37.72%
BenchmarkFmtManyArgs               5933            4742            -20.07%
BenchmarkGobDecode                 53166003        38584736        -27.43%
BenchmarkGobEncode                 37930156        30074874        -20.71%
BenchmarkGzip                      1880638900      1286832100      -31.57%
BenchmarkGunzip                    386343633       292194480       -24.37%
BenchmarkHTTPClientServer          237077          179776          -24.17%
BenchmarkJSONEncode                101731690       73116925        -28.13%
BenchmarkJSONDecode                344655360       241277600       -29.99%
BenchmarkMandelbrot200             28329778        12950809        -54.29%
BenchmarkGoParse                   21670755        16554244        -23.61%
BenchmarkRegexpMatchEasy0_32       557             484             -13.11%
BenchmarkRegexpMatchEasy0_1K       4687            4832            +3.09%
BenchmarkRegexpMatchEasy1_32       539             483             -10.39%
BenchmarkRegexpMatchEasy1_1K       5100            5080            -0.39%
BenchmarkRegexpMatchMedium_32      796             651             -18.22%
BenchmarkRegexpMatchMedium_1K      233099          182047          -21.90%
BenchmarkRegexpMatchHard_32        13202           9897            -25.03%
BenchmarkRegexpMatchHard_1K        401027          303602          -24.29%
BenchmarkRevcomp                   3837679666      2816546600      -26.61%
BenchmarkTemplate                  440608300       324831040       -26.28%
BenchmarkTimeParse                 1460            1019            -30.21%
BenchmarkTimeFormat                1609            1174            -27.04%

benchmark                         old MB/s     new MB/s     speedup
BenchmarkGobDecode                14.44        19.89        1.38x
BenchmarkGobEncode                20.24        25.52        1.26x
BenchmarkGzip                     10.32        15.08        1.46x
BenchmarkGunzip                   50.23        66.41        1.32x
BenchmarkJSONEncode               19.07        26.54        1.39x
BenchmarkJSONDecode               5.63         8.04         1.43x
BenchmarkGoParse                  2.67         3.50         1.31x
BenchmarkRegexpMatchEasy0_32      57.38        66.05        1.15x
BenchmarkRegexpMatchEasy0_1K      218.47       211.91       0.97x
BenchmarkRegexpMatchEasy1_32      59.29        66.21        1.12x
BenchmarkRegexpMatchEasy1_1K      200.76       201.54       1.00x
BenchmarkRegexpMatchMedium_32     1.26         1.53         1.21x
BenchmarkRegexpMatchMedium_1K     4.39         5.62         1.28x
BenchmarkRegexpMatchHard_32       2.42         3.23         1.33x
BenchmarkRegexpMatchHard_1K       2.55         3.37         1.32x
BenchmarkRevcomp                  66.23        90.24        1.36x
BenchmarkTemplate                 4.40         5.97         1.36x

Fixes #10105.

Change-Id: I353cc9fdf922e431821508c9dbbe4d9a85d64bd4
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/8471Reviewed-by: Dave Cheney <dave@cheney.net>

test/nilptr3.go

 // errorcheck -0 -d=nil
-// Fails on ppc64x and arm64 because of incomplete optimization.
-// See issues 9058 and 10105.
-// +build !ppc64,!ppc64le,!arm64
+// Fails on ppc64x because of incomplete optimization.
+// See issues 9058.
+// +build !ppc64,!ppc64le
 
 // Copyright 2013 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style