Uses CheckSmi to force the switch argument to be a Smi, so that it can
be used as an input into a Switch node.

Change-Id: Ibec6beaeebc2168a3f80b86512c70a99d52f2575
Reviewed-on: https://chromium-review.googlesource.com/505621
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45397}