cmd/compile: on amd64 use 32bits copies for 64bits copies of 32bits values

cmd/compile: on amd64 use 32bits copies for 64bits copies of 32bits values

Fixes #76449

This saves a single byte for the REX prefix per OpCopy it triggers on.

Change-Id: I1eab364d07354555ba2f23ffd2f9c522d4a04bd0
Reviewed-on: https://go-review.googlesource.com/c/go/+/731640
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>