This is quite complex since we cannot flush Phi inside the case labels,
we have to do it outside by emitting a lot of manual branches ourselves.
This should be extremely rare, but we need to handle this case.
We made the mistake of registering a dependency on the atomic variable
even if the atomic result was forced to a temporary. There is no need to
register reads from atomic variables like this as we always force atomic
results to a temporary and argument read/writes do not need to be
tracked.
-1 (0xffffffff) literal means the component should be undefined.
Since we cannot express undefined directly, just use a 0 literal in the
appropriate type.
