Reachability is already being tracked in the function body decoder.
This CL adds a DCHECK that the reachability tracked in the SsaEnv
matches the reachability tracked in the function body decoder. Most of
our methods will only be called for reachable code anyway.
For exceptions, we still track reachability explicitly for now in the
wasm graph builder, this can be refactored in the future by improving
reachability tracking for catch blocks in the function body decoder.

If this DCHECK survives fuzzing for a few days, we can remove code that
handles unreachable code in graph-building-interface.cc.

R=herhut@chromium.org

Bug: v8:8423, v8:8611
Change-Id: I0fb375c99497352aad396816566883fe234ca0ac
Reviewed-on: https://chromium-review.googlesource.com/c/1384089Reviewed-by: Stephan Herhut <herhut@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58368}