The underlying issue is that TF Nodes cannot handle input counts
outside the integer range. On an illegal br_table instruction, we
generated a switch node with a control output count >kMaxInt.
Operator::ControlOutputCount turned this into a negative integer later,
leading to a failing DCHECK.
Since such large numbers cannot occur in any valid wasm function anyway,
we just add an additional check to the br table count. There is already
a TODO in the code to change Operator::ControlOutputCount to size_t.

R=ahaas@chromium.org
BUG=chromium:722445

Change-Id: I1975072226e073dee6c8da3b9fa9a050a4695917
Reviewed-on: https://chromium-review.googlesource.com/505496Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45365}