With the infrastructure to support this in place, switch is simply
implemented as a conditional with multiple blocks.  Each block either
ends with a branch to the merge block or the next block, implementing
fallthrough.

Bug: angleproject:4889
Change-Id: I5831531d918ac06648cced7707d1d48ffeb6b1b0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2983559
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>