- JOIN::prepare would have set JOIN::table_count to incorrect value (bad merge of MWL 106)
- optimize_keyuse() would use table-bit as table number
  (the change in optimize_keyuse is also the reason for query plan changes. Not 
   expected to have much effect because only handles cases of no index statistics)
- st_select_lex::register_dependency_item() ignored the fact that some of the 
  selects on the dependency paths could have been merged to their parents (because they 
  were mergeable VIEWs)
- Undo the incorrect fix in Item_subselect::recalc_used_tables(): do not call 
  fix_after_pullout() for Item_subselect::Ref_to_outside members.