pgsql: Ensure that EquivalenceClasses generated from ORDER BY keys

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql/src/backend/optimizer/path:
        pathkeys.c (r1.86 -> r1.87)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/pathkeys.c?r1=1.86&r2=1.87)
    pgsql/src/backend/optimizer/plan:
        createplan.c (r1.231 -> r1.232)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/createplan.c?r1=1.231&r2=1.232)