When decoding a br_table instruction, check each br target only once,
even if it appears several times in the break table.
Also, only mark the merge points as reached after calling the interface
method. This is consistent with the behaviour for br and br_if, and is
needed for implementing Liftoff correctly.

Drive-by: Remove {BreakTo} method which hides trivial functionality
behind a non-trivial method name.
Drive-by^2: Remove redundant reachability tests.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I3f2678c0a6b801b94065dc3e0d452bc2ff82dd50
Reviewed-on: https://chromium-review.googlesource.com/921581Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51321}