cmd/internal/obj/ppc64: consolidate memory classifications

Several classifications exist only to help disambiguate an
implied register (i.e $0/R0 as the implied second register
argument when loading constants, or pseudo-registers used
exclusively by the assembler front-end).

The register determination is folded into getimpliedreg. The
classifications and their related optab entries are removed
or updated.

Change-Id: Iffb167aa9fa57fbc1a537c79fbdfb36cb38f9d95
Reviewed-on: https://go-review.googlesource.com/c/go/+/301789


Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Cherry Zhang <cherryyz@google.com>