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/37290Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>