Type inference too imprecise when approximating union of abstract types

[smarter]

In this example, elem is inferred to be of type Any instead of type X as in Scala 2:

object Test {
  def foo[A <: X, B <: X, X](left: A, right: B): Unit = {
    val elem = if (false) left else right
  }
}

The issue is in TypeOps#approximateUnion, we approximate a union of types using the intersection of the base classes of the types, but that's not correct when the types are bounded by abstract types.