The worker/wrapper transform needs to determine the levity of the result to
determine whether it needs to introduce a lambda to preserve laziness of the
result. For this is previously used isUnliftedType. However, this may fail in
the presence of levity polymorphism.
We now instead use isLiftedType_maybe, assuming that a lambda is needed if the
levity of the result cannot be determined.
Fixes Trac #15186.