SI-7369 Avoid spurious unreachable warnings in patterns.

Unreachability analysis draws on the enumerated domain of types
(e.g sealed subclasses + null, or true/false), and also looks at all
stable identifier patterns tested for equality against the same 'slot'
in a pattern.

It was drawing the wrong conclusions about stable identifier patterns.
Unlike the domain constants, two such values may hold the same value,
so we can't assume that matching X precludes matching Y in the same
slot in a subsequent case.

For example:

    val X: Boolean = true; val Y: Boolean = true
    def m1(t1: Tuple1[Boolean]) = t1 match {
      case Tuple1(true) =>
      case Tuple1(false) =>
      case Tuple1(false) => // correctly unreachable
    }
    def m2(t1: Tuple1[Boolean]) = t1 match {
      case Tuple1(X) =>
      case Tuple1(Y) => // spurious unreachable warning
    }