Optimize joins with similar subqueries

Follow
https://github.com/facebook/presto/issues/1732

A first step, optimize queries of the following shape:

SELECT * 
FROM (
   SELECT k, agg1, agg2
   FROM t
   GROUP BY k
) a
JOIN (
   SELECT k, agg3, agg4
   FROM t
   GROUP BY k
) b
ON (a.k = b.k)

as

SELECT k, agg1, agg2, agg3, agg4
FROM T
GROUP BY k