This is a reland of 0736599a.
This is a reland of 7e1fbe8f.

Original change description:
> [ic] In-place Double -> Tagged transitions
>
> 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/+/1743973
> Reviewed-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}

TBR=verwaest@chromium.org, tebbi@chromium.org

Bug: v8:9606
Change-Id: I2d1b7416064d743582f4983fb868316b7e8a4cf2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1777661Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63499}