This CL adds the beginning of break and continue support. The
conditional versions are not supported, just the non-conditional.
Bug: tint:5
Change-Id: I84418cffd3e29dc011c4313bf9aa3da4833c009f
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/20500
Reviewed-by: David Neto <dneto@google.com>