Instead of having hard to read reinterpret_casts all over the place,
extract this to separate methods.

R=mstarzinger@chromium.org

Bug: v8:9396
Change-Id: Id8d47b5dda8f5b32dedfe9f76c4f526ffcff0674
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1691024Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62594}