Due to recent spec changes, this CL removes the type immediate of
ref.is_null again. Instead we check if the type of the input parameter
is nullable.

R=jkummerow@chromium.org

Bug: v8:10556
Change-Id: If07d30fe4dd27664be7774422573b2ab2b0dfa20
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2247654
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68484}