cmd/internal/obj/ppc64: do not assemble non-constant rotate RLWMI

cmd/internal/obj/ppc64: do not assemble non-constant rotate RLWMI

Unlike RLWNM, the ISA only supports an immediate rotate operand.

Update optab and opirrr to avoid quietly assembling this insn.

Change-Id: I1472a431cb8a870d55d5fff79ab905c4c459f630
Reviewed-on: https://go-review.googlesource.com/c/go/+/449835
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>