The implementation is already quite trivial and in addition in case of
ptr-compr this implementation becomes even simpler and C++ compiler
gets the opportunity to hoist common computation of isolate root.

Tbr: yangguo@chromium.org
Bug: v8:7464, v8:7703
Change-Id: I6e59cd43253c9ade5a0e27025ea7fabecd5b7af7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1647171
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62024}