IMPALA-845: Transfer predicates to 2nd phase merge agg in some cases.

Having predicates need to be transferred to the 2nd phase merge agg
for distinct + non-distinct aggregates without group by.
For distinct + non-distinct aggregates with group by, it is correct
to evaluate the predicates at the 2nd phase (non-merge) agg.

Change-Id: I71d73c4ef92becbb81e142bc0cb5f54e790b1fb5
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1743
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins