Allow ALTER TABLE .. SET NOT NULL to skip provably unnecessary scans.

 Allow ALTER TABLE .. SET NOT NULL to skip provably unnecessary scans.
 
 If existing CHECK or NOT NULL constraints preclude the presence
 of nulls, we need not look to see whether any are present.
 
 Sergei Kornilov, reviewed by Stephen Frost, Ildar Musin, David Rowley,
 and by me.
 
 Discussion: http://postgr.es/m/81911511895540@web58j.yandex.ru