Add test case for EEOP_INNER_SYSVAR/EEOP_OUTER_SYSVAR executor opcodes.

The EEOP_INNER_SYSVAR and EEOP_OUTER_SYSVAR executor opcodes are not
exercised by normal queries, because setrefs.c will resolve the references
to system columns in the scan nodes already. Join nodes refer to them by
their position in the child node's target list, like user columns.

The only place where those opcodes are used, is in evaluating a trigger's
WHEN condition that references system columns. Trigger evaluation abuses
the INNER/OUTER Vars to refer to the OLD and NEW tuples. The code to handle
the opcodes is pretty straightforward, but it seems like a good idea to
have some test coverage for them, anyway, so that they don't get removed or
broken by accident.

Author: Ashutosh Bapat, with some changes by me.
Discussion: https://www.postgresql.org/message-id/CAFjFpRerUFX=T0nSnCoroXAJMoo-xah9J+pi7+xDUx86PtQmew@mail.gmail.com

src/test/regress/expected/triggers.out

@@ -440,6 +440,16 @@ DROP TRIGGER insert_a ON main_table;
 DROP TRIGGER delete_a ON main_table;
 DROP TRIGGER insert_when ON main_table;
 DROP TRIGGER delete_when ON main_table;
+-- Test WHEN condition accessing system columns.
+create table table_with_oids(a int) with oids;
+insert into table_with_oids values (1);
+create trigger oid_unchanged_trig after update on table_with_oids
+	for each row
+	when (new.oid = old.oid AND new.oid <> 0)
+	execute procedure trigger_func('after_upd_oid_unchanged');
+update table_with_oids set a = a + 1;
+NOTICE:  trigger_func(after_upd_oid_unchanged) called: action = UPDATE, when = AFTER, level = ROW
+drop table table_with_oids;
 -- Test column-level triggers
 DROP TRIGGER after_upd_row_trig ON main_table;
 CREATE TRIGGER before_upd_a_row_trig BEFORE UPDATE OF a ON main_table

src/test/regress/sql/triggers.sql

@@ -291,6 +291,16 @@ DROP TRIGGER delete_a ON main_table;
 DROP TRIGGER insert_when ON main_table;
 DROP TRIGGER delete_when ON main_table;
 
+-- Test WHEN condition accessing system columns.
+create table table_with_oids(a int) with oids;
+insert into table_with_oids values (1);
+create trigger oid_unchanged_trig after update on table_with_oids
+	for each row
+	when (new.oid = old.oid AND new.oid <> 0)
+	execute procedure trigger_func('after_upd_oid_unchanged');
+update table_with_oids set a = a + 1;
+drop table table_with_oids;
+
 -- Test column-level triggers
 DROP TRIGGER after_upd_row_trig ON main_table;