This is a reland of 535fd785.
This CL was not the culprit, thus landing unmodified.

Original change's description:
> [wasm] Make DecodeLocals return the number of decoded locals
>
> Currently, when the new locals are not appended to the existing ones,
> there is no way to know how many new locals were defined. This CL
> addresses this issue.
>
> Drive-by: Fix the pc passed to DecodeLocals in OpcodeLength.
> Change-Id: Id9de561a6380b52dcce398301727aa12196c0677
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2567695
> Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
> Reviewed-by: Clemens Backes <clemensb@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#71526}

TBR=manoskouk@chromium.org

Change-Id: I1b2fbe9f6d0a19da9d73202de9f488870e79cd30
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2567704Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71539}