cmd/compile: clean up ssa.Value memory arg usage

cmd/compile: clean up ssa.Value memory arg usage

This change adds a method to replace expressions
of the form

   v.Args[len(v.Args)-1]

so that the code's intention to walk memory arguments
is explicit.

Passes toolstash-check.

Change-Id: I0c80d73bc00989dd3cdf72b4f2c8e1075a2515e0
Reviewed-on: https://go-review.googlesource.com/37757
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>