MDEV-6384: It seems like OPTIMIZER take into account the order of indexes in the table

When ORDER BY ... LIMIT check whether it should switch from index IDX1
to index IDX2, it should not ignore the fact that IDX2 may have a
potential range or ref(const) access.
Istead, it should calculate their costs: there is now a saved range
optimizer cost and code to re-calculate what best_access_path()
calculated for ref(const).

/* in current cost model these two can be very different
numbers unfortunately */

mysql-test/r/innodb_ext_key.result

@@ -1002,7 +1002,7 @@ insert into t2 (b) values (null), (null), (null);
 set optimizer_switch='extended_keys=on';
 explain select a from t1 where b is null order by a desc limit 2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	ref	b	b	9	const	2	Using where; Using filesort
+1	SIMPLE	t1	index	b	PRIMARY	8	NULL	3	Using where
 select a from t1 where b is null order by a desc limit 2;
 a
 3