BUG#826935 Assertion `!table || (!table->read_set || bitmap_is_set(table->read_set, field_index))' failed
- add_ref_to_table_cond() should not just overwrite pre_idx_push_select_cond
  with the contents tab->select_cond.
  pre_idx_push_select_cond exists precisely for the reason that it may contain
  a condition that is a strict superset of what is in tab->select_cond. 
  The fix is to inject generated equality into pre_idx_push_select_cond.