cmd/compile: extend ppc64 MADDLD to match const ADDconst & MULLDconst

cmd/compile: extend ppc64 MADDLD to match const ADDconst & MULLDconst

Fixes #76084

I was focused on restoring the old behavior and fixing the failing
test/codegen/arithmetic.go:MergeMuls2 test.

It is probable this same bug hides elsewhere in this file.

Change-Id: I17f2ee6b97a1e33b8132648d9d750749d006f7e0
Reviewed-on: https://go-review.googlesource.com/c/go/+/715560
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Paul Murphy <paumurph@redhat.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jayanth Krishnamurthy <jayanth.krishnamurthy@ibm.com>