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.cc

@@ -345,6 +345,13 @@ int Explain_node::print_explain_for_children(Explain_query *query,
 }
 
 
+void Explain_select::replace_table(uint idx, Explain_table_access *new_tab)
+{
+  delete join_tabs[idx];
+  join_tabs[idx]= new_tab;
+}
+
+
 Explain_select::~Explain_select()
 {
   if (join_tabs)