[PSE] Remove assumption that top level predicate is union from public interface ...

[PSE] Remove assumption that top level predicate is union from public interface [NFC*]

Note that this doesn't actually cause the top level predicate to become a non-union just yet.

The * above comes from a case in the LoopVectorizer where a predicate which is later proven no longer blocks vectorization due to a change from checking if predicates exists to whether the predicate is possibly false.