This is in preparation for having unfixed slot sizes, where each push
operation will need to record the slot offset of the value.

Bug: v8:9909
Change-Id: I04734d4e67bbae70bdf4351f3afe9d0cc5f3f532
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1913500Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64941}