With only 2GB reservation we can make sure that the heap allocated in
such a way, that all the pointer to it have the most significant bit
in the low halfword set. This allows us to quickly distinguish between
normal pointers and nullptr/sentinel when performing sign-extension
inside decompression.

Bug: chromium:1325007
Change-Id: Ie3a653796bb9dc875ec50103e05cb9aaf55515cf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3793614Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82310}