cmd/compile: optimize a - b == 0 into a == b

cmd/compile: optimize a - b == 0 into a == b

These rules trigger 1141 times during make.bash.

Shrinks a few object files a tiny bit:

name        old object-bytes  new object-bytes  delta
Template          476kB ± 0%        476kB ± 0%  +0.00%  (p=0.008 n=5+5)
Unicode           218kB ± 0%        218kB ± 0%    ~     (all equal)
GoTypes          1.58MB ± 0%       1.58MB ± 0%    ~     (all equal)
Compiler         6.25MB ± 0%       6.25MB ± 0%  -0.00%  (p=0.008 n=5+5)
Flate             304kB ± 0%        304kB ± 0%  -0.01%  (p=0.008 n=5+5)
GoParser          370kB ± 0%        370kB ± 0%    ~     (all equal)
Reflect          1.27MB ± 0%       1.27MB ± 0%    ~     (all equal)
Tar               421kB ± 0%        421kB ± 0%  -0.05%  (p=0.008 n=5+5)
XML               518kB ± 0%        518kB ± 0%    ~     (all equal)

archive/tar benchmarks:

name             old time/op    new time/op    delta
/Writer/USTAR-8    3.97µs ± 1%    3.88µs ± 0%  -2.26%  (p=0.000 n=26+26)
/Writer/GNU-8      4.67µs ± 0%    4.54µs ± 1%  -2.72%  (p=0.000 n=28+27)
/Writer/PAX-8      8.20µs ± 0%    8.01µs ± 0%  -2.32%  (p=0.000 n=29+29)
/Reader/USTAR-8    3.61µs ± 0%    3.54µs ± 1%  -2.04%  (p=0.000 n=25+28)
/Reader/GNU-8      2.27µs ± 2%    2.17µs ± 0%  -4.08%  (p=0.000 n=30+28)
/Reader/PAX-8      7.75µs ± 0%    7.63µs ± 0%  -1.60%  (p=0.000 n=28+28)
[Geo mean]         4.61µs         4.50µs       -2.51%

Change-Id: Ib4dfade5069a7463ccaba073ea91c8213e9714a0
Reviewed-on: https://go-review.googlesource.com/110235
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>
Reviewed-by: Keith Randall <khr@golang.org>