Passive elements have a different binary format, where the contents are
instructions instead of function indexes:

    0xd0 0x0b       -> (ref.null)
    0xd2 var:x 0x0b -> (ref.func x)

Bug: v8:8891
Change-Id: Ie7e8efe7b5acdf99622880dd97d28d3c13744dff
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1497516
Commit-Queue: Ben Smith <binji@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60020}