[dev.ssa] cmd/compile: speed up cse

[dev.ssa] cmd/compile: speed up cse

Construct better initial partitions by recursively comparing values and
their arguments.  This saves one second on compile of arithConst_ssa.go
(4.3s to 3.3s) and shows a 3-5% increase with compilebench.

name       old time/op     new time/op     delta
Template       266ms ± 3%      253ms ± 4%  -5.08%          (p=0.032 n=5+5)
GoTypes        927ms ± 3%      885ms ± 2%  -4.55%          (p=0.016 n=5+5)
Compiler       3.91s ± 3%      3.73s ± 2%  -4.49%          (p=0.008 n=5+5)
MakeBash       31.6s ± 1%      30.5s ± 3%  -3.51%          (p=0.016 n=5+5)

Change-Id: I6ede31ff459131ccfed69531acfbd06b19837700
Reviewed-on: https://go-review.googlesource.com/19838
Reviewed-by: David Chase <drchase@google.com>