Perform Matchable check only if type test is needed

[odersky]

Fixes #16808

[bishabosha]

looks good, I tested some more on my end.

isInstanceOf can also be used as an escape hatch to make an extractor that does not warn:

object Foo:
  def unapply(a: Any): Option[Int] =
    if a.isInstanceOf[Int] then Some(a.asInstanceOf[Int]) else None

def foo(x: Any) = x match
  case Foo(x) => x + 1

should we be concerned about unapply signatures that refine? (not sure how you could validate that anyway, unapply might not even read the parameter)