dart2js cps: Avoid comparing boolean values to boolean constants.

tests/compiler/dart2js/cps_ir/expected/basic_10.js

 // Expectation for test: 
 // main() {
 //   print(new DateTime.now().isBefore(new DateTime.now()));
 // }
 
 function() {
-  var v0 = Date.now() < Date.now(), line = v0 ? "true" : false === v0 ? "false" : String(v0);
+  var line = Date.now() < Date.now() ? "true" : "false";
   if (typeof dartPrint == "function")
     dartPrint(line);
   else if (typeof console == "object" && typeof console.log != "undefined")
     console.log(line);
   else if (!(typeof window == "object")) {
     if (!(typeof print == "function"))
       throw "Unable to print message: " + String(line);
     print(line);
   }
 }