SI-6911, regression in generated case class equality.

Caught out by the different semantics of isInstanceOf and
pattern matching.

  trait K { case class CC(name: String) }
  object Foo extends K
  object Bar extends K
  Foo.CC("a") == Bar.CC("a")

That expression is supposed to be false, and with this
commit it is once again.
Merge pull request #1840 from paulp/issue/6911

SI-6911, regression in generated case class equality.