Do not abstract r0 as a general scratch register -- usage is valid
only in short, localized sequences (since its value may be altered
implicitly by some macro operations).

R=joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=chromium:571370
LOG=N

Review URL: https://codereview.chromium.org/1556643002

Cr-Commit-Position: refs/heads/master@{#33062}