The Field::eq() considered instances of Field_bit that differ only in 
bit_ptr/bit_ofs equal. This caused equality conditions optimization 
(build_equal_items_for_cond()) to make bad field substitutions that result
in wrong predicates. 
Field_bit requires an overloaded eq() function that checks the bit_ptr/bit_ofs
in addition to Field::eq().


mysql-test/r/select.result:
  Bug #18895: BIT values cause joins to fail
  
  - test case
mysql-test/t/select.test:
  Bug #18895: BIT values cause joins to fail
  
  - test case
sql/field.h:
  Bug #18895: BIT values cause joins to fail
  
  - eq() method overloaded for Field_bit