"Early NULL-values filtering for ref access" (attempt2+post-review fixes)
1. update_ref_and_keys() accumulates info about null-rejecting
predicates in in KEY_FIELD::null_rejecting, add_key_part saves
these to KEYUSE.
2. create_ref_for_key copies them to TABLE_REF.
3. add_not_null_conds adds "x IS NOT NULL" to join_tab->select_cond of
appropiate JOIN_TAB members.

Includes code cleanups: 
* add_key_field() params: s/COND/Item_func/ (as only Item_funcs are
passed to it)
* add_key_fields() params: JOIN_TAB *stat removed (wasn't used)


sql/sql_select.cc:
  Fix for BUG#8877: Implementation of 
  "Early NULL-values filtering for ref access"
  1. update_ref_and_keys() accumulates info about null-rejecting
  predicates in in KEY_FIELD::null_rejecting, add_key_part saves
  these to KEYUSE.
  2. create_ref_for_key copies them to TABLE_REF.
  3. add_not_null_conds adds "x IS NOT NULL" to join_tab->select_cond
  of appropiate JOIN_TAB members.
  
  Includes code cleanups: 
  * add_key_field() params: s/COND/Item_func/ (as only Item_funcs are
  passed to it)
  * add_key_fields() params: JOIN_TAB *stat removed (wasn't used)
sql/sql_select.h:
  Fix for BUG#8877: Implementation of 
  "Early NULL-values filtering for ref access" (attempt2)