Changes:
- Remove TypeCheckBranchResult. Change TypeCheckBranch() to return bool.
  Refactor call sites to reflect this (decouple current code
  reachability check from type check).
- Unify TypeCheckBranch(), TypeCheckFallthrough(), and the type-checking
  part of Return() into TypeCheckStackAgainstMerge().
- Make sure all TypeCheck* functions are only called within VALIDATE.
- In graph-builder-interface, rename end_env -> merge_env to reflect
  its function for loops.
- Change expected error messages in some tests.

Change-Id: I857edc18db9c2454ad12d539ffe7a10e96367710
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2839560Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74100}