Also put an UNREACHABLE into an impossible case in
NumberOpFromSpeculativeNumberOp.

R=jarin@chromium.org

Bug: 
Change-Id: I681b7bc58de5038497667cb48fdcd79a73abe1c2
Reviewed-on: https://chromium-review.googlesource.com/819415Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49998}