[wasm] Fix detection of unreachable code

The current Control::unreachable flag served two things at the same
time: It tracked whether the label of that block is reachable (anyone
jumps / falls through to there), and it tracked whether that block was
ended by an unconditional branch or something similar.

It turns out that these two things cannot be tracked with a single
boolean per control struct. This CL introduces a Merge::reached flag,
which tracks whether a merge point was reached so far.
Also, the boolean flag to track unreachability of instructions within a
block is extended to track implicitly unreachable instructions.
According to the spec, a new block inside of unreachable code must be
validated as reachable code again, and also code after a block which
never returns has to be validated. It's not needed to generate code for
such instructions, however.
This new state will be particularly needed for the baseline compiler.

A follow-up CL will avoid calling interface methods for unreachable
code.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I54c97a19121eace65e25c448639330d06f61b1c8
Reviewed-on: https://chromium-review.googlesource.com/715637
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Rossberg <rossberg@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48509}