field='const1' AND field='const2' in some cases

Building multiple equality predicates containing
a constant which is compared as a datetime (with a field)
we should take this fact into account and compare the 
constant with another possible constatns as datetimes 
as well.

E.g. for the
SELECT ... WHERE a='2001-01-01' AND a='2001-01-01 00:00:00'
we should compare '2001-01-01' with '2001-01-01 00:00:00' as
datetimes but not as strings.


mysql-test/r/select.result:
  Fix for bug#49199: Optimizer handles incorrectly: 
  field='const1' AND field='const2' in some cases
    - test result.
mysql-test/t/select.test:
  Fix for bug#49199: Optimizer handles incorrectly: 
  field='const1' AND field='const2' in some cases
    - test case.
sql/item_cmpfunc.cc:
  Fix for bug#49199: Optimizer handles incorrectly: 
  field='const1' AND field='const2' in some cases
    - adding a constant to Item_equal compare it as
  a datetime value with stored one if there's a 
  date[time] field in a equality predicate.
sql/item_cmpfunc.h:
  Fix for bug#49199: Optimizer handles incorrectly: 
  field='const1' AND field='const2' in some cases
    - adding a constant to Item_equal compare it as
  a datetime value with stored one if there's a 
  date[time] field in a equality predicate.
sql/sql_select.cc:
  Fix for bug#49199: Optimizer handles incorrectly: 
  field='const1' AND field='const2' in some cases
    - adding a constant to Item_equal compare it as
  a datetime value with stored one if there's a 
  date[time] field in a equality predicate.