Loose index scan does the grouping so the temp table does 
not need to do it, even when sorting.
Fixed by checking if the grouping is already done before
doing sorting and grouping in a temp table and do only 
sorting.


mysql-test/r/group_min_max.result:
  Bug #32268: test case
mysql-test/t/group_min_max.test:
  Bug #32268: test case
sql/sql_select.cc:
  Bug #32268: don't group in the temp table if already done