cmd/internal/obj/ppc64: improve RLWNM encoding

cmd/internal/obj/ppc64: improve RLWNM encoding

If the rotate argument is the constant 0, rlwnm may be generated
instead of rlwinm. In all reasonable cases, this is OK as R0 should
hold 0. However, this could be problematic in some cases when
writing PPC64 assembly.

This consolidates the RLWNM and RLWMI optab entries. Invalid RLWMI
usage is still rejected, however the error will be be slightly
different. The invalid usage will be caught in oprrr instead of oplook.

Change-Id: I9958bd24660fea5f8fc9e3e50d51daa7349e3206
Reviewed-on: https://go-review.googlesource.com/c/go/+/527275
Reviewed-by: Heschi Kreinick <heschi@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Run-TryBot: Paul Murphy <murp@ibm.com>