Improve Hash/Merge Join estimate accuracy when all predicates are Hash/Merge clauses

Hi hackers,

The planner currently calls approx_tuple_count() to estimate
hashjointuples and mergejointuples. That makes sense when
joinrestrictinfo contains additional clauses beyond the hash/merge
equality list. But if all join restriction clauses are exactly those
hash/merge clauses, the estimate already computed in
path->jpath.path.rows is usually more accurate (and free).

This patch reuses path->jpath.path.rows in that case and skips
approx_tuple_count().

Regression results
==================

                     | actual | approx | estimate
---------------------+--------+--------+-------
join.sql q1          |      5 |      1 |     5
join.sql q2          | 10 000 |      1 | 10 000
join.sql q3          |      5 |      5 |     5
join.sql q4          |      5 |      5 |     5
join.sql q5          |      5 |      1 |     5
partition_join q1    |    200 |      1 |   200
partition_join q2    |     42 |     84 |    84
partition_join q3    |      8 |      1 |     8
postgres_fdw         |   2001 |   1001 |  2001
select_parallel q1   |      0 |  5 000 | 5 000
updatable_views q1   |      2 |      2 |   423

Two cases get worse: select_parallel.sql and updatable_views.sql.

Looking forward to your feedback!

--
Best regards,
Ilia Evdokimov,
Tantor Labs LLC.