Update matching_candidates_in_table() to treat all conditions similar

Fixed also that the 'with_found_constraint parameter' to
matching_candidates_in_table() is as documented: It is now true only
if there is a reference to a previous table in the WHERE condition for
the current examined table (as it was originally documented)

Changes in test results:
- Filtered was 25% smaller for some queries (expected).
- Some join order changed (probably because the tables had very few rows).
- Some more table scans, probably because there would be fewer returned
  rows.
- Some tests exposes a bug that if there is more filtered rows, then the
  cost for table scan will be higher. This will be fixed in a later commit.

mysql-test/main/distinct.result

@@ -173,9 +173,9 @@ INSERT INTO t2 values (1),(2),(3);
 INSERT INTO t3 VALUES (1,'1'),(2,'2'),(1,'1'),(2,'2');
 explain SELECT distinct t3.a FROM t3,t2,t1 WHERE t3.a=t1.b AND t1.a=t2.a;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t2	index	a	a	4	NULL	5	Using index; Using temporary
-1	SIMPLE	t1	eq_ref	PRIMARY	PRIMARY	4	test.t2.a	1	Using where
+1	SIMPLE	t1	ALL	PRIMARY	NULL	NULL	NULL	4	Using where; Using temporary
 1	SIMPLE	t3	ref	a	a	5	test.t1.b	2	Using index
+1	SIMPLE	t2	index	a	a	4	NULL	5	Using where; Using index; Using join buffer (flat, BNL join)
 SELECT distinct t3.a FROM t3,t2,t1 WHERE t3.a=t1.b AND t1.a=t2.a;
 a
 1
@@ -523,7 +523,7 @@ EXPLAIN SELECT DISTINCT t1_1.a, t1_1.b FROM t1 t1_1, t1 t1_2
 WHERE t1_1.a = t1_2.a;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1_1	ALL	PRIMARY	NULL	NULL	NULL	3	Using temporary
-1	SIMPLE	t1_2	eq_ref	PRIMARY	PRIMARY	4	test.t1_1.a	1	Using index; Distinct
+1	SIMPLE	t1_2	index	PRIMARY	PRIMARY	4	NULL	3	Using where; Using index; Using join buffer (flat, BNL join)
 EXPLAIN SELECT a FROM t1 GROUP BY a;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	3	Using index