Re: upper planner path-ification

Once we support to add aggregation path during path consideration,
we need to pay attention morphing of the final target-list according
to the intermediate path combination, tentatively chosen.
For example, if partial-aggregation makes sense from cost perspective;
like SUM(NRows) of partial COUNT(*) AS NRows instead of COUNT(*) on
billion rows, planner also has to adjust the final target-list according
to the interim paths. In this case, final output shall be SUM(), instead
of COUNT().