SI-6771 Alias awareness for checkableType in match analysis.

Failure to dealias the type of the scrutinee led the pattern
matcher to incorrectly reason about the type test in:

    type Id[X] = X; (null: Id[Option[Int]]) match { case Some(_) => }

Before, `checkableType` returned `Id[?]`, now it returns `Some[?]`.

Review by @adriaanm