Move IS [NOT] NULL handling from BRIN support functions

Move IS [NOT] NULL handling from BRIN support functions

The handling of IS [NOT] NULL clauses is independent of an opclass, and
most of the code was exactly the same in both minmax and inclusion. So
instead move the code from support procedures to the AM.

This simplifies the code - especially the support procedures - quite a
bit, as they don't need to care about NULL values and flags at all. It
also means the IS [NOT] NULL clauses can be evaluated without invoking
the support procedure.

Author: Tomas Vondra 
Author: Nikita Glukhov 
Reviewed-by: Nikita Glukhov 
Reviewed-by: Mark Dilger 
Reviewed-by: Alexander Korotkov 
Reviewed-by: Masahiko Sawada 
Reviewed-by: John Naylor 
Discussion: https://postgr.es/m/c1138ead-7668-f0e1-0638-c3be3237e812@2ndquadrant.com