fixbug: IS [NOT] NULL operator was not generated correctly.
bugfix: gpuscan works incorrectly if order of variables appeared in WHERE clause is different from table declaration
bugfix: kern_varlena_to_toast() incorrectly calculate the offset to put

arithmetic_stairlike_add() returns sum of values owned by workitems
that has less or equal local-id of the called, but kern_varlena_to_toast
assumes it returns only sum of values owned by workitems less than itself.
So, it leads incorrect offset value to put toast datum.