Fix bug with passes filtering too many cases:

((), Nil()) passes {
  case () => Cons(0, Nil())
}

was true(!) because Nil </: Cons and Cons </: Nil which caused the case
to filtered out silently.

src/main/scala/leon/purescala/Constructors.scala

@@ -145,7 +145,7 @@ object Constructors {
 
     resType match {
       case Some(tpe) =>
-        casesFiltered.filter(c => isSubtypeOf(c.rhs.getType, tpe) || isSubtypeOf(tpe, c.rhs.getType))
+        casesFiltered.filter(c => typesCompatible(c.rhs.getType, tpe))
       case None =>
         casesFiltered
     }