add device-quals pull-up on bulkloading
pull-up outer-quals on gpupreagg

This enhancement allows to pull-up qualifiers of outer relation on
GpuPreAgg if underlying GpuScan has no host-only qualifiers.
It reduces number of opencl interaction and improve data density
of PCI-E data transfer.

postgres=# explain select cat, avg(x) from t0 where x < 50.0 group by cat;
INFO:  Agg cost = 379300.23..379300.31 nrows (10048333 => 179)
                                    QUERY PLAN
-----------------------------------------------------------------------------------
 HashAggregate  (cost=379300.31..379300.23 rows=7 width=12)
   Group Key: cat
   ->  Custom (GpuPreAgg)  (cost=10335.27..379299.33 rows=179 width=112)
         Bulkload: On
         Device Filter: (x < 50::double precision)
         ->  Custom (GpuScan) on t0  (cost=0.00..266667.00 rows=20000024 width=12)
 Planning time: 2.130 ms
(7 rows)
cosmetic change: kills warnning because of unused variables