lwa cannot encode unaligned displacements.  lwz is acceptable here
since the shift right will perform the sign-extension on PPC64 for us.

R=arv@chromium.org, dstence@us.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1134913004

Cr-Commit-Position: refs/heads/master@{#28517}