Changes:
- Remove redundant argument from PopControl(), FallThruTo();
- Rename FallThruTo() -> FallThrough();
- Do not Kill() the environment at control end in
  graph-builder-interface, as this is not needed.
- Move some things around and remove dead code.

Change-Id: Ia2e2fb5c3a60c32838d42e5916691b38642b30bc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2830792
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74004}