Ensure that EquivalenceClasses generated from ORDER BY keys contain proper

RelabelType nodes when the sort key is binary-compatible with the sort
operator rather than having exactly its input type.  We did this correctly
for index columns but not sort keys, leading to failure to notice that
a varchar index matches an ORDER BY request.  This requires a bit more work
in make_sort_from_pathkeys, but not anyplace else that I can find.
Per bug report and subsequent discussion.