Isolate::shared_isolate() was used in many locations to check for the
shared heap feature. Now that we also have shared_space_isolate()
checking shared_isolate() isn't sufficient anymore.

This CL replaces many invocations of this method with either
has_shared_heap() or shared_heap_isolate(). These methods work for
both shared_isolate() and shared_space_isolate(). As soon as we remove
the shared isolate we can remove them again.

Bug: v8:13267
Change-Id: I68a3588aca2a12e204450c2b99635dd158d12111
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3899316Reviewed-by: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83260}