This is simply done with OpKill.  If-else blocks now check if the block
is already terminated (through return, discard, continue or break)
before adding the branch to merge block.

Bug: angleproject:4889
Change-Id: I713286cf38e40f9048486d2914a4355ddbc686ac
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2953369
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>