The degenerate br_table case should be handled specially only in the
graph building consumer. There it is necessary for avoiding the
construction of a degenerate Switch node, which would cause a DCHECK
error in instruction selection.
For other backends, like the baseline compiler, we should handle it as
a br_table, because the signature is different to a br.

Drive-by: Fix redundant validation.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Ia430b6d251eb1323848977388ed95a112f8c76f7
Reviewed-on: https://chromium-review.googlesource.com/715616Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48494}