dart2js cps: Hoist unsafe expressions from loop entry.

tests/compiler/dart2js/cps_ir/expected/codeUnitAt_2.js

 // Expectation for test: 
 // // Bounds checking
 // foo(s) {
 //   var sum = 0;
 //   for (int i = 0; i < s.length; i++) sum += s.codeUnitAt(i);
 //   return sum;
 // }
 // main() {
 //   print(foo('ABC'));
 //   print(foo('Hello'));
 // }
 
 function() {
   var v0 = "ABC".length, sum = 0, i = 0;
   for (; i < v0; sum += "ABC".charCodeAt(i), ++i)
     ;
   P.print(sum);
+  v0 = "Hello".length;
   sum = 0;
-  for (i = 0; i < "Hello".length; sum += "Hello".charCodeAt(i), ++i)
+  for (i = 0; i < v0; sum += "Hello".charCodeAt(i), ++i)
     ;
   P.print(sum);
 }