Don't flag match type aliases as unreducible

If `M[_]` is a match type alias, it was rejected for being an unreducible
application of wildcard types. It is now accepted. I believe that is sound
- this is siply an unreducible match type. The situation is not the same
as `F[_]` where `F` is an abstract type that can be refined in several ways
in subclasses.

Fixes scala#9999

Author: odersky