cmd/compile: ensure we don't reuse temporary register

cmd/compile: ensure we don't reuse temporary register

Before this CL, we could use the same register for both a temporary
register and for moving a value in the output register out of the way.

Fixes #71857

Change-Id: Iefbfd9d4139136174570d8aadf8a0fb391791ea9
Reviewed-on: https://go-review.googlesource.com/c/go/+/651221
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>