Fix ADD IF NOT EXISTS used in conjunction with ALTER TABLE ONLY

The flag for IF NOT EXISTS was only being passed down in the normal
recursing case. It's been this way since originally added in 9.6 in
commit 2cd40ad so backpatch back to 9.6.

Author: gsstark

src/backend/commands/tablecmds.c

@@ -4042,7 +4042,7 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
 		case AT_AddColumnToView:	/* add column via CREATE OR REPLACE VIEW */
 			address = ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,
 									  false, false, false,
-									  false, lockmode);
+									  cmd->missing_ok, lockmode);
 			break;
 		case AT_AddColumnRecurse:
 			address = ATExecAddColumn(wqueue, tab, rel, (ColumnDef *) cmd->def,

src/test/regress/expected/alter_table.out

@@ -3306,6 +3306,9 @@ ALTER TABLE test_add_column
 ALTER TABLE test_add_column
 	ADD COLUMN c2 integer; -- fail because c2 already exists
 ERROR:  column "c2" of relation "test_add_column" already exists
+ALTER TABLE ONLY test_add_column
+	ADD COLUMN c2 integer; -- fail because c2 already exists
+ERROR:  column "c2" of relation "test_add_column" already exists
 \d test_add_column
           Table "public.test_add_column"
  Column |  Type   | Collation | Nullable | Default 
@@ -3316,6 +3319,9 @@ ERROR:  column "c2" of relation "test_add_column" already exists
 ALTER TABLE test_add_column
 	ADD COLUMN IF NOT EXISTS c2 integer; -- skipping because c2 already exists
 NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
+ALTER TABLE ONLY test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer; -- skipping because c2 already exists
+NOTICE:  column "c2" of relation "test_add_column" already exists, skipping
 \d test_add_column
           Table "public.test_add_column"
  Column |  Type   | Collation | Nullable | Default 

src/test/regress/sql/alter_table.sql

@@ -2054,9 +2054,13 @@ ALTER TABLE test_add_column
 \d test_add_column
 ALTER TABLE test_add_column
 	ADD COLUMN c2 integer; -- fail because c2 already exists
+ALTER TABLE ONLY test_add_column
+	ADD COLUMN c2 integer; -- fail because c2 already exists
 \d test_add_column
 ALTER TABLE test_add_column
 	ADD COLUMN IF NOT EXISTS c2 integer; -- skipping because c2 already exists
+ALTER TABLE ONLY test_add_column
+	ADD COLUMN IF NOT EXISTS c2 integer; -- skipping because c2 already exists
 \d test_add_column
 ALTER TABLE test_add_column
 	ADD COLUMN c2 integer, -- fail because c2 already exists