Port 3db1d4a5

Original Commit Message:

    Currently, atomic operations assume the default to be 32-bit
    operations, fix opcode names for differentiation between 32/64-bit
    operations.

R=gdeepti@chromium.org, joransiu@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Change-Id: I79c195ce7474f329ccb2446fad1f81bab41e7329
Reviewed-on: https://chromium-review.googlesource.com/937921Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#51580}