fix NOT VALID NOT NULL with ALTER COLUMN SET IDENTITY

hi.

The attached patch makes the last two statements below fail.
CREATE TABLE notnull_tbl1 (a int, b int);
INSERT INTO notnull_tbl1 VALUES (NULL, 1), (300, 3);
ALTER TABLE notnull_tbl1 ADD CONSTRAINT nn NOT NULL a NOT VALID; -- ok
ALTER TABLE notnull_tbl1 ALTER COLUMN a ADD GENERATED ALWAYS AS
IDENTITY; --error
ALTER TABLE notnull_tbl1 ADD CONSTRAINT nn NOT NULL a NOT VALID,
  ALTER COLUMN a ADD GENERATED ALWAYS AS IDENTITY; --error


but in another case,
CREATE TABLE notnull_tbl1 (a int, b int);
INSERT INTO notnull_tbl1 VALUES (NULL, 1), (300, 3);
ALTER TABLE notnull_tbl1 ADD CONSTRAINT nn NOT NULL a NOT VALID;

I am not so sure the below two statements should error out or not?
ALTER TABLE notnull_tbl1 ADD COLUMN c int GENERATED BY DEFAULT AS
IDENTITY, ADD CONSTRAINT nn NOT NULL c NOT VALID;
ALTER TABLE notnull_tbl1 ADD COLUMN d serial, ADD CONSTRAINT nn NOT
NULL c NOT VALID;