dart2js cps: More aggressive operator specialization.

tests/compiler/dart2js/cps_ir/expected/argument_refinement_num_9.js

 // Expectation for test: 
 // main() {
 //   var x = int.parse('1233');
 //   var y = int.parse('1234');
 //   print(x is num);
 //   print(y is num);
 //   print(x ^ y);
 //   print(x is num);
 //   print(y is num); // will be compiled to `true` if we know the type of `y`.
 // }
 
 function() {
   var x = P.int_parse("1233", null, null), y = P.int_parse("1234", null, null);
   P.print(typeof x === "number");
   P.print(typeof y === "number");
-  P.print(J.$xor$n(x, y));
+  if (typeof x !== "number" || typeof y !== "number")
+    return J.$xor$n(x, y);
+  P.print((x ^ y) >>> 0);
   P.print(true);
   P.print(true);
 }