Fix PK detection on inherited tables.

Fix PK detection on inherited tables.

We don't allow UPDATEs or DELETEs on tables without primary key but the
code for detecting if table has primary key didn't work correctly when
the UPDATE/DELETE was run on parent table which had primary key but the
command has propagated to an inherited table didn't have primary key.

In passing, fix the primary key detection for a less common use case:
WITH foo AS ( UPDATE table_without_pk ... RETURNING * )
INSERT INTO table_with_pk SELECT * FROM foo;