The current compression scheme defines isomorphism with respect to
relational operations (i.e. the relational operators preserve their
results on the set of compressed pointers).

In addition, provide overloads for nullptr/sentinel.

Bug: chromium:1325007
Change-Id: I476a1c59e92f5210e26142320eb03802bd11ea51
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3758143Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81706}