... rather than as a suffix.

This avoids reading previous values which in turn will make things
simpler from the pointer compression point of view in a sense that
this is a step towards replacing UnalignedSlot with MaybeObjectSlot.

Bug: v8:8794, v8:8562
Change-Id: I9a9b4a01f73b8058074d337b7e9e9f75fa1c9de0
Reviewed-on: https://chromium-review.googlesource.com/c/1456037
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59402}