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

Bug: v8:6532
Change-Id: Idc7df4e191f54b125271b067891e0a1df07008a4
Reviewed-on: https://chromium-review.googlesource.com/924333Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51532}