This addresses a TODO and makes the CAST explicit at the use sites,
which in turn makes it easier to understand the implications there.

Bug: v8:9183
Change-Id: I5b285a5a0ba6b754b4de9d60ff6e18c2e807bf98
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1649569
Auto-Submit: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62079}