This is a reland of 4a037f87

Changes compared to original change: None. This seems not to create
problems after all.

Original change's description:
> [wasm][bug] Fix a couple of bugs in validation of unreachable code
>
> 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}

Change-Id: Id620f7fb6677b772b0dcfd38108256384db44439
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2905598Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74677}