This adds support for if blocks to Liftoff.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Iac7f3ec6526e1259b447ab6ec794a8a1c7ae05c3
Reviewed-on: https://chromium-review.googlesource.com/853501
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50430}