[dev.ssa] cmd/compile: PPC64, add cmp->bool, some shifts, hmul

Includes hmul (all widths)
compare for boolean result and simplifications
shift operations plus changes/additions for implementation
(ORN, ADDME, ADDC)

Also fixed a backwards-operand CMP.

Change-Id: Id723c4e25125c38e0d9ab9ec9448176b75f4cdb4
Reviewed-on: https://go-review.googlesource.com/25410


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>