With recent changes to the anyref proposal, null refs now have a type
immediate which declares the type of a null ref constant. Likewise,
the RefIsNull instruction is type aware now. This CL addresses these
proposal changes now.

R=jkummerow@chromium.org

Bug: v8:10556
Change-Id: I810dfa3a4ab4389afc9639f897cee5d43e9b62cb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2215172
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68141}