cmd/compile/ssa: more aggressive constant folding

cmd/compile/ssa: more aggressive constant folding

Add rewrite rules that canonicalize the location
of constants in expressions, and fold conststants
that appear in operations that can be trivially
reassociated.

After this change, the compiler constant-folds
expressions like "4 + x - 1" and "4 & x & 1"

Benchmarks affected on darwin/amd64:

name                     old time/op    new time/op    delta
FmtFprintfInt-8            82.1ns ± 1%    81.7ns ± 1%  -0.46%  (p=0.023 n=8+9)
FmtFprintfIntInt-8          122ns ± 2%     120ns ± 2%  -1.48%  (p=0.047 n=10+10)
FmtManyArgs-8               493ns ± 0%     486ns ± 1%  -1.37%  (p=0.000 n=8+10)
Gzip-8                      230ms ± 0%     229ms ± 1%  -0.46%  (p=0.001 n=10+9)
HTTPClientServer-8         74.5µs ± 1%    73.7µs ± 1%  -1.11%  (p=0.000 n=10+10)
JSONDecode-8               51.7ms ± 0%    51.9ms ± 1%  +0.42%  (p=0.017 n=10+9)
RegexpMatchEasy0_32-8      82.6ns ± 1%    81.7ns ± 0%  -1.02%  (p=0.000 n=9+8)
RegexpMatchMedium_32-8      121ns ± 1%     120ns ± 1%  -1.48%  (p=0.001 n=10+10)
Revcomp-8                   426ms ± 1%     400ms ± 1%  -6.16%  (p=0.000 n=10+10)
TimeFormat-8                330ns ± 1%     327ns ± 0%  -0.82%  (p=0.000 n=10+10)

name                     old speed      new speed      delta
Gzip-8                   84.4MB/s ± 0%  84.8MB/s ± 1%  +0.47%  (p=0.001 n=10+9)
JSONDecode-8             37.6MB/s ± 0%  37.4MB/s ± 1%  -0.42%  (p=0.016 n=10+9)
RegexpMatchEasy0_32-8     387MB/s ± 1%   392MB/s ± 0%  +1.06%  (p=0.000 n=9+8)
RegexpMatchMedium_32-8   8.21MB/s ± 1%  8.34MB/s ± 1%  +1.58%  (p=0.000 n=10+9)
Revcomp-8                 597MB/s ± 1%   636MB/s ± 1%  +6.57%  (p=0.000 n=10+10)

Change-Id: Ie37ff91605b76a984a8400dfd1e34f50bf61c864
Reviewed-on: https://go-review.googlesource.com/37290
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>