Changes:
- SetBlockType now instantiates the block's start merge with values of
  the correct type in unreachable code.
- EnsureStackArguments now keeps the existing stack values and moves
  them over the new bottom values.
- Drop stack size validation in Drop().
- Add new tests in unreachable-validation.js.

Change-Id: Ie68b3d9abb0a41d1623d4a123fb526e71941c4e7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2902733
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74650}