BUG #16185: Trigger fires twice and with wrong TG_OP when updating partion key on partitioned table

The following bug has been logged on the website:

Bug reference:      16185
Logged by:          Henry Hinze
Email address:      [email protected]
PostgreSQL version: 12.1
Operating system:   Ubuntu 16.04.6 LTS
Description:        

If a trigger is defined on a partitioned table and the partition key is
updated that the row is moved to another partition, then the trigger is
fired twice first with TG_OP = 'DELETE' followed by TG_OP = 'INSERT'.  As a
user I would expect that the trigger fires only once with TG_OP =
'UPDATE'.

To reproduce:
****************
create table t (id int, p_key int) partition by list (p_key);

create table t1 partition of t for values in (1);
create table t2 partition of t for values in (2);

insert into t (id,p_key) values (1,1);

create or replace function f()
    RETURNS TRIGGER
    LANGUAGE plpgsql
AS
$function$
BEGIN
    RAISE NOTICE 'TG_OP: %', TG_OP;
    RETURN NULL;
END;
$function$;

CREATE TRIGGER f_trig
    AFTER INSERT or UPDATE or DELETE
    ON t
    FOR EACH ROW
EXECUTE PROCEDURE f();

postgres=# update t set p_key = 2 where id = 1;
NOTICE:  TG_OP: DELETE
NOTICE:  TG_OP: INSERT
UPDATE 1