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}