Break early when creating Memtuple binding from TupleDesc.
Break early when creating Memtuple binding from TupleDesc.

MemtupleBinding is column aligned by 4 or 8 bytes.
It can be set to 8 if we find a atrribute's attalign is 'd' (8 bytes on
most machines), there is no need to check other attributes.
Add some comments about how to compute null bitmap entries space.

Authored-by: Mingli Zhang <63af0d80c53af00dcbd1248b19f1a4b34c35c682@gmail.com>
xinzweb/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,50wu/gpdb,50wu/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb