Distinguish move/load/store ops.  Unify some of this code a bit.

Reduces Mandelbrot slowdown with SSA from 58% to 12%.

Change-Id: I3276eaebcbcdd9de3f8299c79b5f25c0429194c4
Reviewed-on: https://go-review.googlesource.com/18677
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>