Code after discard, return, break and continue is discarded with this
change.  This simplifies SPIR-V generation as no code is allowed after a
branch instruction and no special handling is necessary after this
change.

Bug: angleproject:4889
Change-Id: Ife9c8de8a6d0db9d682561daf44366aad9b1cf61
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2957811Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>