Changes:
- Rename PassThrough -> Forward in function-body-decoder.
- Introduce IsHeapSubtypeOf in subtyping.
- Do not push a redundant bottom value in br_on_null, remove
  fallthrough. Also, improve code structure.
- Update a couple of comments.

Bug: v8:7748
Change-Id: I8d23cd3829c5504156ace595f8ac86c511c9f5e1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2611250
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72034}