Fixed a bug that rows hint can be omitted on some condition.

Fixed a bug that rows hint can be omitted on some condition.

A hint for higer level may be canceled by unnecessary reestimation
caused by a rows hint for lower level.

During making a joinrel, if there's a rows hint just matches the
joinrelids, and has already been applied on an earlier try for the same
joinrelids, and if there's another rows hint which is applicable on
either component of the join, the change of rownums made earlier is
canceled by the 'another' hint causing reestimation of the joinrel.
Finally the effect of the former hint disappears.