Add support for stores that transition to writable data fields,
based on the BeginRegion/FinishRegion mechanism for atomic regions
in the scheduler.

This is early work and still a bit rough around the edges, and similar
to regular stores, we don't support transitioning stores to double
fields yet.

R=jarin@chromium.org
BUG=v8:4470
LOG=n

Review URL: https://codereview.chromium.org/1406153010

Cr-Commit-Position: refs/heads/master@{#31645}