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/union.result

@@ -565,7 +565,7 @@ explain (select * from t1 where a=1 and b=10) union (select straight_join t1.a,t
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 2	UNION	t1	index	PRIMARY	PRIMARY	4	NULL	4	Using index
-2	UNION	t2	eq_ref	PRIMARY	PRIMARY	4	test.t1.a	1	Using index
+2	UNION	t2	index	PRIMARY	PRIMARY	4	NULL	4	Using where; Using index; Using join buffer (flat, BNL join)
 NULL	UNION RESULT	<union1,2>	ALL	NULL	NULL	NULL	NULL	NULL	
 set @@optimizer_use_condition_selectivity=@save_optimizer_use_condition_selectivity,@@optimizer_switch=@save_optimizer_switch;
 explain (select * from t1 where a=1) union (select * from t1 where b=1);