Fix disassembly of atomic operations for the inspector.

BUG=v8:6842,v8:6532

Change-Id: I3701b55c28b10561d1726e2c0b9fe2e1b2c76b8e
Reviewed-on: https://chromium-review.googlesource.com/703468
Commit-Queue: Brad Nelson <bradnelson@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Reviewed-by: Ben Smith <binji@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48324}