Description:
------------
is_null() is 4x slower than ===null

Reproduce code:
---------------
<?
 $t=microtime(1);
 for($z=0;$z<1e7;$z++){
  if(is_null(1)){}
 }
 print(microtime(1)-$t);print('<br/>');

// ---

 $t=microtime(1);
 for($z=0;$z<1e7;$z++){
  if(1===null){}
 }
 print(microtime(1)-$t);print('<br/>');
?>

Expected result:
----------------
is_null() is faster like ===null

Actual result:
--------------
is_null() is 4x slower than ===null