Only emit typeguards if we think they are valuable.

tests/language/src/ListDoubleIndexInLoop2Test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 // Dart test program for testing arrays.
 
 bar() => true;
 
 tata() => 1.5;
 
 // The type propagation in Dart2Js wrongly took the intersection of all incoming
 // types in a loop-phi. In this case the back-edge brought type 'number' which,
 // combined with 'integer' (i = 0) was narrowed to 'integer'. As a result no
 // check was inserted for the list access.
 foo(a) {
   var i;
-  if (bar()) {
-    // t's desired type is conflicting. Once it is used as array receiver. And
-    // once as integer. The backward propagation thus can't decide.
-    // The forward declaration, however, will assign type num.
-    var t = 0 + tata();
-    i = t;
-    if (!bar()) t[0];
-  } else {
-    i = 0;
+  for (int j = 0; j < 1; j++) {
+    if (bar()) {
+      // t's desired type is conflicting. Once it is used as array receiver. And
+      // once as integer. The backward propagation thus can't decide.
+      // The forward declaration, however, will assign type num.
+      var t = 0 + tata();
+      i = t;
+      if (!bar()) t[0];
+    } else {
+      i = 0;
+    }
+    // The phi, combining the two 'i's must reach the conclusion that i is of
+    // type num and therefore needs a check before accessing the array.
+    a[i];
   }
-  // The phi, combining the two 'i's must reach the conclusion that i is of
-  // type num and therefore needs a check before accessing the array.
-  a[i];
 }
 
 main() {
   Expect.throws(() => foo([1, 2]));
 }