The function body decoder already tracks reachability and only calls
the backend for reachable code. Thus the checks in the TF backend can
be skipped.
Note that there was already a DCHECK in place that checks whether
{SsaEnv::reached()} always returns true. This was to give fuzzers a
chance to prove the reachability analysis in the function body decoder
wrong. Since this did not happen, we can not remove the {reached()}
method from SsaEnv, since it always returned true anyway.

R=titzer@chromium.org

Bug: v8:8423, v8:8611
Change-Id: I2336aa7a092e381e3d94d87051b6e14db0dbab99
Reviewed-on: https://chromium-review.googlesource.com/c/1415493Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58871}