Add additional nodes to capture places where control flow is changed
(if, switch, for, short-circuiting op, function call), and use these
to show the actual point at which control flow became non-uniform as a
result of a non-uniform value.
Do this recursively, to capture cases where control flow becomes
non-uniform after a function call statement.
Bug: tint:880
Change-Id: Ied92d690f98a5c11a1892eef500a50d0f123943d
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/89862
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>