The interface will now only be called for reachable code, hence the
check in the graph builder is not needed any more. We DCHECK instead.

R=titzer@chromium.org

Change-Id: I24ef96d62d6a5cda18d9efefcbd089d5f324f624
Reviewed-on: https://chromium-review.googlesource.com/716176Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48675}