... in order to improve performance of decompression code on C++ side
(because computation of isolate root from isolate pointer becomes a trivial
reinterpret cast) and measure the impact separately from other ptr-compr
changes.

Bug: v8:9353
Change-Id: I36906cef2968355411ee944d97625ecd2652646b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1835550Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64115}