Instruction selector computes the range of case statement labels
to choose between a table or lookup based switch. We need to special
case this when there are no case statements.

Bug: chromium:794825
Change-Id: I46ef57d17f5e2b99a3570f7f3c4ff06e75d78fab
Reviewed-on: https://chromium-review.googlesource.com/830013Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Mythri Alle <mythria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50153}