dart2js cps: More aggressive operator specialization.

tests/compiler/dart2js/cps_ir/expected/argument_refinement_21.js

 // Expectation for test: 
 // main() {
 //   var x = int.parse('1233');
 //   var y = int.parse('1234');
 //   print(x / 2);
 //   print(x is num);
 //   print(y is num);
 //   print(x.compareTo(y));
 //   print(y is num);
 // }
 
 function() {
   var x = P.int_parse("1233", null, null), y = P.int_parse("1234", null, null), v0 = typeof y === "number";
-  P.print(J.$div$n(x, 2));
+  P.print(x / 2);
   P.print(true);
   P.print(v0);
   if (!v0)
     throw H.wrapException(H.argumentErrorValue(y));
   P.print(x < y ? -1 : x > y ? 1 : x === y ? x === 0 ? 1 / x < 0 === (y === 0 ? 1 / y < 0 : y < 0) ? 0 : 1 / x < 0 ? -1 : 1 : 0 : isNaN(x) ? isNaN(y) ? 0 : 1 : -1);
   P.print(true);
 }