Adjust costs for doing index scan in cost_group_min_max()

The idea is that when doing a tree dive (once per group), we need to
compare key values, which is fast.  For each new group, we have to
compare the full where clause for the row.
Compared to original code, the cost of group_min_max() has slightly
increased which affects some test with only a few rows.
main.group_min_max and main.distinct have been modified to show the
effect of the change.

The patch also adjust the number of groups in case of quick selects:
- For simple WHERE clauses, ensure that we have at least as many groups
  as we have conditions on the used group-by key parts.
  The assumption is that each condition will create at least one group.
- Ensure that there are no more groups than rows found by quick_select

Test changes:
- For some small tables there has been a change of
  Using index for group-by -> Using index for group-by (scanning)
  Range -> Index and Using index for group-by -> Using index

mysql-test/main/partition_range.test

@@ -17,6 +17,7 @@ drop table if exists t1, t2;
 CREATE TABLE t1 (a INT,b INT,KEY a (a,b));
 INSERT INTO `t1` VALUES (0,580092),(3000,894076),(4000,805483),(4000,913540),(6000,611137),(8000,171602),(9000,599495),(9000,746305),(10000,272829),(10000,847519),(12000,258869),(12000,929028),(13000,288970),(15000,20971),(15000,105839),(16000,788272),(17000,76914),(18000,827274),(19000,802258),(20000,123677),(20000,587729),(22000,701449),(25000,31565),(25000,230782),(25000,442887),(25000,733139),(25000,851020);
 
+SELECT COUNT(*) from t1 where a IN (10000, 1000000, 3000);
 EXPLAIN SELECT a, MAX(b) FROM t1 WHERE a IN (10000, 1000000, 3000) GROUP BY a;
 
 alter table t1 partition by hash(a) partitions 1;