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/delete.result

@@ -205,13 +205,13 @@ insert into t3 values (1,1),(2,1),(1,3);
 select * from t1,t2,t3 where t1.a=t2.a AND t2.b=t3.a and t1.b=t3.b;
 a	b	a	b	a	b
 1	1	1	1	1	1
-2	1	2	2	2	1
 1	3	1	1	1	3
+2	1	2	2	2	1
 explain select * from t1,t2,t3 where t1.a=t2.a AND t2.b=t3.a and t1.b=t3.b;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	
-1	SIMPLE	t2	ref	PRIMARY	PRIMARY	4	test.t1.a	1	Using index
-1	SIMPLE	t3	eq_ref	PRIMARY	PRIMARY	8	test.t2.b,test.t1.b	1	Using index
+1	SIMPLE	t2	index	PRIMARY	PRIMARY	8	NULL	3	Using where; Using index; Using join buffer (flat, BNL join)
+1	SIMPLE	t3	index	PRIMARY	PRIMARY	8	NULL	3	Using where; Using index; Using join buffer (incremental, BNL join)
 delete t2.*,t3.* from t1,t2,t3 where t1.a=t2.a AND t2.b=t3.a and t1.b=t3.b;
 select * from t3;
 a	b