[wasm-gc] Fix/simplify unreachable code validation

- Maintain the correct stack in unreachable code for all type casts.
- Compute the correct type for the pushed stack value for ref.cast.
- Check if current_code_reachable_and_ok_ instead of checking the
  popped values' types against bottom.
- Add unit tests.

Bug: v8:7748
Change-Id: I02c26f526060f40884c4ff1e541315f71d8ad90a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2928191


Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74912}