We now remember whether the memory was 64 bit, in in this case force the
index value to be an i64 instead of an i32.

This is only the decoding part of this change. TurboFan and Liftoff will
have to be fixed separately to handle the i64 values correctly.

R=manoskouk@chromium.org

Bug: v8:10949
Change-Id: Ia504e7eb5a2a55caf8dfdbd0833481ef590c55bf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2461239
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70473}