This introduces an AllocateMutableHeapNumberStub for the boxed double
field case, where we need to allocate a box in case of a transitioning
store first. We cannot use our inline allocations for this currently,
because mutable HeapNumber objects have certain alignment constraints,
and I don't want to mess up Allocate/AllocateInNewSpace eagerly.

Also refactor the PropertyAccessInfoFactory slightly to split the long
methods into simpler parts.

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

Review URL: https://codereview.chromium.org/1419173007

Cr-Commit-Position: refs/heads/master@{#31695}