Return type restriction on `NilableCast` (`as?`)

[straight-shoota]

I noticed that the return type of x.as?(T) is always T | Nil. In some cases however, could be further reduced. This could allow removal of some code paths that would never be reached anyway.

• If typeof(x) <= T, the result is always T.
• If typeof(x) has no intersection with Y, the result is always Nil.

I don't see any particular reason why the return type should not be restricted as much as possible, but maybe I'm missing something.