[MASSMAIL]Incorrect handling of IS [NOT] NULL quals on inheritance parents

In b262ad440e we introduced an optimization that drops IS NOT NULL quals
on a NOT NULL column, and reduces IS NULL quals on a NOT NULL column to
constant-FALSE.  I happened to notice that this is not working correctly
for traditional inheritance parents.  Traditional inheritance parents
might have NOT NULL constraints marked NO INHERIT, while their child
tables do not have NOT NULL constraints.  In such a case, we would have
problems when we have removed redundant IS NOT NULL restriction clauses
of the parent rel, as this could cause NULL values from child tables to
not be filtered out, or when we have reduced IS NULL restriction clauses
of the parent rel to constant-FALSE, as this could cause NULL values
from child tables to not be selected out.  As an example, consider

create table p (a int);
create table c () inherits (p);

alter table only p alter a set not null;

insert into c values (null);

-- The IS NOT NULL qual is droped, causing the NULL value from 'c' to
-- not be filtered out
explain (costs off) select * from p where a is not null;
       QUERY PLAN
-------------------------
 Append
   ->  Seq Scan on p p_1
   ->  Seq Scan on c p_2
(3 rows)

select * from p where a is not null;
 a
---

(1 row)