- Decode logic for atomic operations
 - Implementations for I32AtomicAdd, I32AtomicAdd8U, I32AtomicAdd16U,
I32AtomicSub, I32AtomicSub8U, I32AtomicSub16U
 - cctest value helpers for Uint16/Uint8 types

R=binji@chromium.org, bbudge@chromium.org, bradnelson@chromium.org

BUG=v8:6532

Change-Id: I710ee8ef566c5e33866afdf5b47375c2ea6fdbe6
Reviewed-on: https://chromium-review.googlesource.com/595241Reviewed-by: Bill Budge <bbudge@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Reviewed-by: Ben Smith <binji@chromium.org>
Commit-Queue: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47102}