BUG #16258: exec_simple_query does not call ProcessCompletedNotifies after CommitTransactionCommand

The following bug has been logged on the website:

Bug reference:      16258
Logged by:          RekGRpth
Email address:      rekgrpth@gmail.com
PostgreSQL version: 12.2
Operating system:   Docker alpine edge
Description:

to reproduse

psql -c "begin;select pg_notify('channel', concat_ws(' = ', 'a',
clock_timestamp()::text));commit;select pg_sleep(10);begin;select
pg_notify('channel', concat_ws(' = ', 'b',
clock_timestamp()::text));commit;select now(), clock_timestamp();"