This adds some additional rules to improve loads and
stores for ppc64x.

Change-Id: I96b99c3a0019e6ac5393910c086f58330a04fc5a
Reviewed-on: https://go-review.googlesource.com/27354
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>