With the separate flagalloc pass, it should be fine to
allow CSE of control values.  The worst that can happen
is that the comparison gets un-CSEd by flagalloc.

Fix bug in flagalloc where flag restores were getting
clobbered by rematerialization during register allocation.

Change-Id: If476cf98b69973e8f1a8eb29441136dd12fab8ad
Reviewed-on: https://go-review.googlesource.com/17760Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Keith Randall <khr@golang.org>