cmd/compile: rewrite cmov(x, x, cond) into x

cmd/compile: rewrite cmov(x, x, cond) into x

I don't think branchelim will intentionally generate theses.
But at the time where branchelim is generating them they might different,
and through opt process they become the same value.

Change-Id: I4a19f1db14c08057b7e782a098f4c18ca36ab7fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/690519
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Mark Freeman <mark@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>