cmd/compile: let MADD/MSUB combination happen more often on arm64

cmd/compile: let MADD/MSUB combination happen more often on arm64

We have a single-instruction x+y*z op. Unfortunately x can't be
a constant, so the rule that builds them doesn't apply in that case.

This CL handles x+(c+y*z) by reordering to c+(x+y*z) so x is
in the right place.

Change-Id: Ibed621607d49da70474128e20991e0c4630ebfad
Reviewed-on: https://go-review.googlesource.com/c/go/+/629858
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>