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/join.test

@@ -253,7 +253,7 @@ CREATE TABLE t2 (
   PRIMARY KEY  (id)
 ) ENGINE=MyISAM;
 INSERT INTO t2 VALUES (1,'s1'),(2,'s2'),(3,'s3'),(4,'s4'),(5,'s5');
-select t1.*, t2.*  from t1, t2 where t2.id=t1.t2_id limit 2;
+select straight_join t1.*, t2.*  from t2, t1 where t2.id=t1.t2_id limit 2;
 drop table t1,t2;
 
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