With no more MutableHeapNumber, we can make Double -> Tagged transitions
in-place, at the cost of an extra map check when accessing double fields
to make sure they are still doubles.

Bug: v8:9606
Change-Id: I74ff39ed6fba62ee223cd37dfe761f7d73020e1c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1743973Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63374}