Bug #40925: Equality propagation takes non indexed attribute

Query execution plans and execution time of queries like

  select a, b, c from t1
    where a > '2008-11-21' and b = a limit 10

depended on the order of equality operator parameters:
"b = a" and "a = b" are not same. 


An equality propagation algorithm has been fixed:
the substitute_for_best_equal_field function should not
substitute a field for an equal field if both fields belong
to the same table.


mysql-test/r/select.result:
  Added test case for bug #40925.
mysql-test/t/select.test:
  Added test case for bug #40925.
sql/item.cc:
  Bug #40925: Equality propagation takes non indexed attribute
  
  An equality propagation algorithm has been fixed:
  the substitute_for_best_equal_field function should not 
  substitute a field for an equal field if both fields belong
  to the same table.