cmd/internal/obj/ppc64: refactor maskgen64 usage

cmd/internal/obj/ppc64: refactor maskgen64 usage

Converting and verifying a bitmask can be done much quicker
and simpler. Since this touches the MD-form opcodes, cleanup
their encoding too.

Change-Id: I9b1c1fdc4c9622e489ff6cf8181c5b647afae7c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/534017
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Paul Murphy <murp@ibm.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>