s390x does not require duffzero/duffcopy since it has
storage-to-storage instructions that can copy/clear up to 256
bytes at a time.

peep contains several new passes to optimize instruction
sequences that match s390x instructions such as the
compare-and-branch and load/store multiple instructions.

copyprop and subprop have been extended to work with moves that
require sign/zero extension. This work could be ported to other
architectures that do not used sized math however it does add
complexity and will probably be rendered unnecessary by ssa in
the near future.

Change-Id: I1b64b281b452ed82a85655a0df69cb224d2a6941
Reviewed-on: https://go-review.googlesource.com/20873
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bill O'Farrell <billotosyr@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>