when executed in version 5

Zero fill is a field attribute only. So we can't always 
propagate  constants for zerofill fields : the values and 
expression results don't have that flag.

Fixed by converting the const value to a string and 
using that in const propagation when the context allows it. 
Disable const propagation for fields with ZEROFILL flag in
all the other cases.


mysql-test/r/compare.result:
  Bug #31887: test case
mysql-test/t/compare.test:
  Bug #31887: test case
sql/item.cc:
  Bug #31887: If the context allows conversion
  of an int constant to a zero-filled string constant
  put the string constant instead of the int constant
  when doing const propagation
sql/mysql_priv.h:
  Bug #31887: a macro to get all the Field_num
  descendant fields.