STREX does not permit using the same register for the value to store
and the place where the result is returned. Also the code was wrong
anyhow if the first store failed.

Fixes #22248

Change-Id: I96013497410058514ffcb771c76c86faa1ec559b
Reviewed-on: https://go-review.googlesource.com/70911
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>