MDEV-406: ANALYZE $stmt: get ANALYZE work for subqueries

- "ANALYZE $stmt" should discard select's output, but it should still
  evaluate the output columns (otherwise, subqueries in select list
  are not executed)
- SHOW EXPLAIN's code practice of calling JOIN::save_explain_data()
  after JOIN::exec() is disastrous for ANALYZE, because it resets
  all counters after the first execution. It is stopped
  = "Late" test_if_skip_sort_order() calls explicitly update their part
    of the query plan.
  = Also, I had to rewrite I_S optimization to actually have optimization
    and execution stages.

sql/sql_explain.h

@@ -129,6 +129,12 @@ public:
     join_tabs[n_join_tabs++]= tab;
     return false;
   }
+  
+  /*
+    This is used to save the results of "late" test_if_skip_sort_order() calls
+    that are made from JOIN::exec
+  */
+  void replace_table(uint idx, Explain_table_access *new_tab);
 
 public:
   int select_id;