Improve GADT usage tracing for MatchType reduction

[Linyxus]

#15851 merges a nice fix for issue #15743. However, in some cases the usage of GADT constraints is not recorded so the GADT cast is not inserted. This PR tries to cover more cases where GADTused should be set.

[Linyxus]

Note that this PR still cannot cover all the cases. For example, the following code still fails the tree checker:

trait Tag[S]
case class TupTag[A, T <: Tuple]() extends Tag[A *: T]

def tupleId[T <: Tuple](x: T): x.type = x

def foo[S](x: S, ev: Tag[S]) = ev.match {
  case _: TupTag[a, t] =>
    val t1: t = tupleId(x.tail)
}

It seems that the cause of the failure is related to type inference. In the subtype trace I cannot find a place where we can record the usage of GADT constraints for match type reduction. I am not 100% sure about what happens.

[dwijnand]

Very nice