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

Change-Id: Id894d177c7fe8cc3f451be80c273b50daee91378
Reviewed-on: https://chromium-review.googlesource.com/1113544
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54012}