cmd/compile: on amd64, use flag result of x instead of doing (TEST x x)

cmd/compile: on amd64, use flag result of x instead of doing (TEST x x)

So we can avoid using a TEST where it isn't needed.

Currently only implemented for ADD{Q,L}const.

Change-Id: Ia9c4c69bb6033051a45cfd3d191376c7cec9d423
Reviewed-on: https://go-review.googlesource.com/c/go/+/669875
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>