MDEV-13186: main.win failure post MDEV-12336

During statement preparation st_order::item gets set to a value in
ref_ptr_array. During statement execution we were overriding that value,
causing subsequent checks for window functions to return true.

Whenever we do any setting from ref_ptr_array, make sure to always
store the value in all_fields as well.

For function items containing window functions, as MDEV-12336 has
discovered, we don't need to create a separate Item_direct_ref or
Item_aggregate_ref as they will be computed directly from the top-level
item once the window function argument columns are computed.

mysql-test/r/win.result

@@ -3085,3 +3085,15 @@ select max(id), rank() over (order by max(id)) from t1 where id < 3;
 max(id)	rank() over (order by max(id))
 2	1
 drop table t1;
+#
+# main.win failure post MDEV-12336
+#
+create table t(a decimal(35,10), b int);
+insert into t values (1, 10), (2, 20), (3, 30);
+prepare stmt from "SELECT (CASE WHEN sum(t.a) over (partition by t.b)=1 THEN 1000 ELSE 300 END) AS a FROM t";
+execute stmt;
+a
+1000
+300
+300
+drop table t;

mysql-test/t/win.test

@@ -1877,3 +1877,13 @@ select count(max(id)) over (order by max(id)) from t1 where id < 3;
 select max(id), rank() over (order by max(id)) from t1 where id < 3;
 
 drop table t1;
+
+--echo #
+--echo # main.win failure post MDEV-12336
+--echo #
+create table t(a decimal(35,10), b int);
+insert into t values (1, 10), (2, 20), (3, 30);
+
+prepare stmt from "SELECT (CASE WHEN sum(t.a) over (partition by t.b)=1 THEN 1000 ELSE 300 END) AS a FROM t";
+execute stmt;
+drop table t;

sql/item.cc

@@ -1920,6 +1920,9 @@ void Item::split_sum_func2(THD *thd, Ref_ptr_array ref_pointer_array,
       point to the temporary table.
     */
     split_sum_func(thd, ref_pointer_array, fields, split_flags);
+    if (type() == FUNC_ITEM) {
+      return;
+    }
   }
   else
   {
@@ -1979,9 +1982,6 @@ void Item::split_sum_func2(THD *thd, Ref_ptr_array ref_pointer_array,
                                         &ref_pointer_array[el], 0, name))))
       return;                                   // fatal_error is set
   }
-  else if (type() == FUNC_ITEM && 
-           ((Item_func *) this)->with_window_func)
-    return;
   else
   {
     if (!(item_ref= (new (thd->mem_root)