cmd/compile: fix rewrite rules for multiply/add

cmd/compile: fix rewrite rules for multiply/add

x - (y - c) == (x - y) + c, not (x - y) - c. Oops.

Fixes #70481

Change-Id: I0e54d8e65dd9843c6b92c543ac69d69ee21f617c
Reviewed-on: https://go-review.googlesource.com/c/go/+/630397
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>
Reviewed-by: Jakub Ciolek <jakub@ciolek.dev>
Auto-Submit: Keith Randall <khr@golang.org>