This CL simplifies the approach to HeapNumbers in concurrent
compilation. We'll only create a HeapNumberRef for immutable
HeapNumbers -- this means that we don't need to validate the read
of the value with a compilation dependency check. Mutable
HeapNumbers are handled differently (the value is read for
constant folding, and protected with a constant field dependency).

This CL includes 2 reverts:
Revert "[compiler] Make HeapNumberRef background serialized"
Revert "[compiler] Fix endianness issue when reading HeapNumber"

Bug: v8:7790
Change-Id: I24e65583b787c214b917e96e789d711c2a7c9694
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2891576
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74567}