Trivial fix: Make test_if_cheaper_ordering() use actual_rec_per_key()

Discovered this while working on MDEV-34720: test_if_cheaper_ordering()
uses rec_per_key, while the original estimate for the access method
is produced in best_access_path() by using actual_rec_per_key().

Make test_if_cheaper_ordering() also use actual_rec_per_key().
Also make several getter function "const" to make this compile.
Also adjusted the testcase to handle this (the change backported from
11.0)

mysql-test/main/subselect_innodb.result

@@ -554,6 +554,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 #
 # MDEV-6081: ORDER BY+ref(const): selectivity is very incorrect (MySQL Bug#14338686)
 #
+insert into t2 select seq,seq,seq from seq_10000_to_11000;
 alter table t2 add key2 int;
 update t2 set key2=key1;
 alter table t2 add key(key2);

mysql-test/main/subselect_innodb.test

@@ -558,7 +558,9 @@ from
 --echo #
 --echo # MDEV-6081: ORDER BY+ref(const): selectivity is very incorrect (MySQL Bug#14338686)
 --echo #
+--source include/have_sequence.inc
 
+insert into t2 select seq,seq,seq from seq_10000_to_11000;
 alter table t2 add key2 int;
 update t2 set key2=key1;
 alter table t2 add key(key2);

sql/sql_select.cc

@@ -30391,7 +30391,8 @@ test_if_cheaper_ordering(const JOIN_TAB *tab, ORDER *order, TABLE *table,
     else
     {
       const KEY *ref_keyinfo= table->key_info + ref_key;
-      refkey_rows_estimate= ref_keyinfo->rec_per_key[tab->ref.key_parts - 1];
+      refkey_rows_estimate=
+        (ha_rows)ref_keyinfo->actual_rec_per_key(tab->ref.key_parts - 1);
     }
     set_if_bigger(refkey_rows_estimate, 1);
   }

sql/sql_statistics.h

@@ -472,7 +472,7 @@ class Index_statistics
 
   bool avg_frequency_is_inited() { return avg_frequency != NULL; }
 
-  double get_avg_frequency(uint i)
+  double get_avg_frequency(uint i) const
   {
     return (double) avg_frequency[i] / Scale_factor_avg_frequency;
   }

sql/structs.h

@@ -166,7 +166,7 @@ typedef struct st_key {
   engine_option_value *option_list;
   ha_index_option_struct *option_struct;                  /* structure with parsed options */
 
-  double actual_rec_per_key(uint i);
+  double actual_rec_per_key(uint i) const;
 
   bool without_overlaps;
   /*

sql/table.cc

@@ -10034,7 +10034,7 @@ uint TABLE_SHARE::actual_n_key_parts(THD *thd)
 }  
 
 
-double KEY::actual_rec_per_key(uint i)
+double KEY::actual_rec_per_key(uint i) const
 { 
   if (rec_per_key == 0)
     return 0;