Several SSA ops will always behave identically regardless of target
architecture, so handle those within gc/ssa.go instead.

Passes toolstash-check -all.

Change-Id: I54d514e80ab86723e44332a5a38e3054cbca8c5d
Reviewed-on: https://go-review.googlesource.com/37931
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>