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/subselect_exists2in.result

@@ -52,7 +52,7 @@ explain extended
 SELECT * FROM t1 WHERE EXISTS ( SELECT a FROM t3 WHERE t3.b = t1.a);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	PRIMARY	t1	index	aa	aa	4	NULL	2	100.00	Using index
-1	PRIMARY	t3	ALL	bb	NULL	NULL	NULL	2	100.00	Using where; FirstMatch(t1); Using join buffer (flat, BNL join)
+1	PRIMARY	t3	ALL	bb	NULL	NULL	NULL	2	75.00	Using where; FirstMatch(t1); Using join buffer (flat, BNL join)
 Warnings:
 Note	1276	Field or reference 'test.t1.a' of SELECT #2 was resolved in SELECT #1
 Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` semi join (`test`.`t3`) where `test`.`t3`.`b` = `test`.`t1`.`a`