Port a94d6d6e

Original commit message:
    The mode requires an extra register, and since we aren't supporting
    it now, we can dispense with it.

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

Review URL: https://codereview.chromium.org/1559013005

Cr-Commit-Position: refs/heads/master@{#33118}