On-heap typed arrays contain HeapObject value in |base_pointer| field
and an offset in |external_pointer| field. When pointer compression is
enabled we want to combine decompression with the offset addition.
In order to do that we add an isolate root to the external_pointer value
and therefore the data pointer computation can is a simple addition of
a (potentially sign-extended) |base_pointer| loaded as Tagged_t value
and an |external_pointer| value.

Bug: v8:9706
Change-Id: Id5c546c353c81fb25e3598921bc78165d10a9c44
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1807369Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63874}