cmd/compile: reuse sort helpers

cmd/compile: reuse sort helpers

sort.Sort's argument always escapes.
cse generates many calls to sort.Sort.
Set up a single escaping variable
and re-use it across loops.

name       old alloc/op     new alloc/op     delta
Template       40.7MB ± 0%      40.2MB ± 0%  -1.24%        (p=0.000 n=15+15)
Unicode        33.4MB ± 0%      33.3MB ± 0%  -0.09%        (p=0.000 n=15+15)
GoTypes         121MB ± 0%       119MB ± 0%  -1.48%        (p=0.000 n=14+15)
Compiler        474MB ± 0%       465MB ± 0%  -1.94%        (p=0.000 n=14+15)

name       old allocs/op    new allocs/op    delta
Template         405k ± 0%        394k ± 0%  -2.64%        (p=0.000 n=15+15)
Unicode          350k ± 0%        350k ± 0%  -0.14%        (p=0.000 n=14+15)
GoTypes         1.21M ± 0%       1.18M ± 0%  -3.07%        (p=0.000 n=15+14)
Compiler        4.37M ± 0%       4.18M ± 0%  -4.39%        (p=0.000 n=15+15)

Change-Id: I68cf56dafa0f3ea778826eea19908bd761556154
Reviewed-on: https://go-review.googlesource.com/32220
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>