Avoid reversing user-given order of WHERE clauses while attaching clauses

Avoid reversing user-given order of WHERE clauses while attaching clauses
to specific base or join RelOptInfo nodes during planning.  This preserves
the more-intuitive behavior of 7.0.* --- if you write an expensive clause
(such as a sub-select) last, it should get evaluated last.  Someday we
ought to try to have some intelligence about the order of evaluation of
WHERE clauses, but for now we should not override what the user wrote.