Remove excessive update_used_tables() calls

In this code in sql_select.cc:

static COND *build_equal_items(JOIN *join, COND *cond,

                               COND_EQUAL *inherited,

                               List<TABLE_LIST> *join_list,

                               bool ignore_on_conds,

                               COND_EQUAL **cond_equal_ref,

                               bool link_equal_fields)

{

  THD *thd= join->thd;

  COND_EQUAL *cond_equal= 0;

  if (cond) 

  {

    cond= build_equal_items_for_cond(thd, cond, inherited, link_equal_fields);

    cond->update_used_tables();

  ...

the call for cond->update_used_tables() is not needed in most cases.

Look into all "return" in build_equal_items_for_cond().
update_used_tables() is either not needed at all, like in case when "new Item_int" is returned,
or update_used_tables() is already called inside build_equal_items_for_cond().

This task is to move the call for code->update_used_tables() from build_equal_items() into build_equal_items_for_cond(), and then to get rid of the unnecessary calls in the cases that don't really need it.