This CL implements Liftoff support for struct.get/set,
struct.new_with_rtt, rtt.canon, and ref.is_null, which
is enough to make the first testcase pass.

Bug: v8:7748
Change-Id: Id09e9872d2126127192c852b3cb6d57ff9417582
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2584951
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71744}