For the reference types anyref, anyfunc, and nullref, there exist
sub-typing rules. The spec says

A reference type reftype1 matches a reference type reftype2 if and only
if:

* Either both reftype1 and reftype2 are the same.
* Or reftype1 is nullref.
* Or reftype2 is anyref.

This CL introduces the type nullref for ref-null, and implements the
sub-typing rules in the function-body-decoder.

Note that because of the sub-typing check validation performance may
regresses. In that case we can optimize the sub-typing check.

R=titzer@chromium.org, clemensh@chromium.org

Bug: v8:7581
Change-Id: I55bab72a109f3374da3770d141b0fc8067aad8b2
Reviewed-on: https://chromium-review.googlesource.com/c/1430061
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59043}