These will tight-loop scanning primitives.

BUG=v8:6092

Change-Id: I9bf0f1952755bbede3c545c45fe2c4a210548171
Reviewed-on: https://chromium-review.googlesource.com/647526Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47788}