This allows to turn two field in the LiftoffAssembler into constants.

R=thibaudm@chromium.org

Bug: v8:12425
Change-Id: Ie39ca73d6bb704b42bd449eed984f426e69deeb5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3585956Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79989}