Move instanceof checks in typer to allow typing more GADTs

[smarter]

The fact that IsInstanceOfEvaluator is a separate phase is nice for modularity but unfortunately it means that even though this works:

class Term[A]
case class Number(val n: Int) extends Term[Int]
object Test {
  def f[B](t: Term[B]): B = t match {
    case y @ Number(_) => y.n
  }
}

this doesn't (example from SLS 8.3):

class Term[A]
class Number(val n: Int) extends Term[Int]
object Test {
  def f[B](t: Term[B]): B = t match {
    case y: Number => y.n
  }
}

The first example typechecks because we explicitly check if the scrutinee is compatible with the pattern type : https://github.com/lampepfl/dotty/blob/04b0e85c70de36fd7b5a4b3ffd1b8add0963303d/src/dotty/tools/dotc/typer/Applications.scala#L758, this check is done with GADTflexible set which allows B to be constrained to be equal to Int inside the case, if this is not done the method will not typecheck, but for patterns of the form x: T, this check is not done in typer but in IsInstanceOfEvaluator which is too late.
The optimizations can still be done in a separate phase but the checks need to be done in typer.

[DarkDimius]

Fixed in #1309