The way we use FinishRegion for transitioning stores makes them eligible
for elimination by TypedOptimization, which is unintended and removes
the atomicity of the transitioning stores. This is a quickfix to ensure
that we don't remove the FinishRegion nodes during TypedOptimization;
the real fix is probably to have separate region operators for value
(producing) regions (i.e. allocations) and for effect-only regions (i.e.
transitioning stores).

R=jarin@chromium.org
BUG=v8:5303

Review-Url: https://codereview.chromium.org/2293023003
Cr-Commit-Position: refs/heads/master@{#39033}