cmd/compile/internal/ssa: combine more shift and masking on PPC64

cmd/compile/internal/ssa: combine more shift and masking on PPC64

Investigating binaries, these patterns seem to show up frequently.

Change-Id: I987251e4070e35c25e98da321e444ccaa1526912
Reviewed-on: https://go-review.googlesource.com/c/go/+/583302
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>