Fix #830: Compiler hangs on implicit search with singleton &/|

[odersky]

In fact we get a deep subtype recursion when compileing i830.scala. The problem
goes away once we make use of the fact that the intersection of two singleton
types which are not subtypes of each other is empty. Review by @smarter

[odersky]

@smarter How does it look now?

[smarter]

The change LGTM but note that the following still causes an infinite loop:

object C {
  val a: Int = 1
  val b: Int = 2
  val c: Int = 2

  trait X[T]
  implicit def u[A, B]: X[A | B] = new X[A | B] {}
  def y[T](implicit x: X[T]): T = ???
  val x: a.type & b.type | b.type & c.type = y
}