The reg-reg version compiled to PSRAW, not PSRLW (arithmetic
instead of logical shift right).

Fixes #13010.

Change-Id: I69a47bd83c8bbe66c7f8d82442ab45e9bf3b94fb
Reviewed-on: https://go-review.googlesource.com/16168Reviewed-by: Ian Lance Taylor <iant@golang.org>