This is a reland of f0bcbc90.
A few casts were still wrong.

Original change's description:
> Reland "Introduce MutableHeapNumber class."
>
> This is a reland of 40ac6b18, which
> was incorrect due to a bad merge.
>
> Original change's description:
> > Introduce MutableHeapNumber class.
> >
> > V8 knows heap numbers and mutable heap numbers. They have
> > difference instance types, but in C++ code we've used the
> > same class for both (HeapNumber). Confusingly, however,
> > IsHeapNumber would return false for mutable heap numbers,
> > while HeapNumber::cast would succeed.
> >
> > This CL adds a separate class MutableHeapNumber and
> > eliminates the confusing behavior.
> >
[...]
> TBR=bmeurer@chromium.org
> TBR=ulan@chromium.org
>
> Change-Id: I3af1014c949821dfac0754a3e48c65ce1bad1ad1
> Reviewed-on: https://chromium-review.googlesource.com/1114539
> Reviewed-by: Georg Neis <neis@chromium.org>
> Commit-Queue: Georg Neis <neis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#54022}

Change-Id: I19a33da4b6abcd445b528a84d4f56ba1964d337b
Reviewed-on: https://chromium-review.googlesource.com/1114100
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54027}