Port 0d59772b

Original commit message:
    for the special case where the same register is used as both left and
    right input.

R=jkummerow@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1698903002

Cr-Commit-Position: refs/heads/master@{#34004}