use ref not range for ... WHERE key IN (val)

mysql-test/r/range.result

@@ -255,7 +255,7 @@ t1	ref	y	y	5	const	1	Using where
 t2	range	x	x	5	NULL	2	Using where
 explain select count(*) from t1 where x in (1);
 table	type	possible_keys	key	key_len	ref	rows	Extra
-t1	range	x	x	5	NULL	1	Using where; Using index
+t1	ref	x	x	5	const	1	Using where; Using index
 explain select count(*) from t1 where x in (1,2);
 table	type	possible_keys	key	key_len	ref	rows	Extra
 t1	range	x	x	5	NULL	2	Using where; Using index

sql/sql_select.cc

@@ -1590,12 +1590,14 @@ add_key_fields(JOIN_TAB *stat,KEY_FIELD **key_fields,uint *and_level,
     // BETWEEN or IN
     if (cond_func->key_item()->type() == Item::FIELD_ITEM)
       add_key_field(key_fields,*and_level,
-		    ((Item_field*) (cond_func->key_item()))->field, 0,
+		    ((Item_field*) (cond_func->key_item()))->field,
 #ifndef TO_BE_REMOVED_IN_4_1
                     /* special treatment for IN. Not necessary in 4.1 */
+                    cond_func->argument_count() == 1,
                     cond_func->arguments()      + (cond_func->functype() != Item_func::IN_FUNC),
                     cond_func->argument_count() - (cond_func->functype() != Item_func::IN_FUNC),
 #else
+                    cond_func->argument_count() == 2,
                     cond_func->arguments()+1, cond_func->argument_count()-1,
 #endif
                     usable_tables);