The issue here is that for a window function in the ORDER BY clause, we were not
creating an extra field in the temporary table for the window function
(which is contained in an expression).
So a call to split_sum_func is added to handle this case

Also we need to update all items that contain a window function
in the temp table during window function computation as filesort would need
these values to be updated to calculate the ORDER BY clause of the select.