cmd/compile: improve multiplication strength reduction

cmd/compile: improve multiplication strength reduction

Use an automatic algorithm to generate strength reduction code.
You give it all the linear combination (a*x+b*y) instructions in your
architecture, it figures out the rest.

Just amd64 and arm64 for now.

Fixes #67575

Change-Id: I35c69382bebb1d2abf4bb4e7c43fd8548c6c59a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/626998
Reviewed-by: Jakub Ciolek <jakub@ciolek.dev>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>