dart2js cps: Hoist unsafe expressions from loop entry.

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

Index: tests/compiler/dart2js/cps_ir/expected/codeUnitAt_2.js
diff --git a/tests/compiler/dart2js/cps_ir/expected/codeUnitAt_2.js b/tests/compiler/dart2js/cps_ir/expected/codeUnitAt_2.js
index 58429f15741e3341c464dc7514bd997f494c27f7..73e278b67792408f658af7253931f00d575c1d04 100644
--- a/tests/compiler/dart2js/cps_ir/expected/codeUnitAt_2.js
+++ b/tests/compiler/dart2js/cps_ir/expected/codeUnitAt_2.js
@@ -15,8 +15,9 @@ function() {
   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);
 }