This allows to easily introduce new nodes with untagged represenation.
It also speeds up the is_untagged_value check.

Bug: v8:7700
Change-Id: Ie391d32ae7742dbad481674de262050c0d564ee6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3581773
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Auto-Submit: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79941}