Last week's patch for make_sort_from_pathkeys wasn't good enough: it has

Last week's patch for make_sort_from_pathkeys wasn't good enough: it has
to be able to discard top-level RelabelType nodes on *both* sides of the
equivalence-class-to-target-list comparison, since make_pathkey_from_sortinfo
might either add or remove a RelabelType.  Also fix the latter to do the
removal case cleanly.  Per example from Peter.