Classify CFG edges:
- loop backedge
- a structured exit:
- loop break
- loop continue
- selection break
- fallthrough
- forward (any of the rest)
Also error out when there should have been a merge instruction.
(More than one unique fallthrough or forward edge).
Includes lots of tests.
Bug: tint:3
Change-Id: I70f27680bdf098213056522abf04ac58a6b478ab
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/20481
Reviewed-by: dan sinclair <dsinclair@google.com>