Make sure that we don't assign phis before its uses.

tests/language/loop_exchange2_test.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.
+
+// This program tripped dart2js.
+main() {
+  var a = 1;
+  var b = 2;
+  var c = 3;
+  var d = 4;
+  var e = 5;
+  for (int i = 0; i < 2; i++) {
+    if (i == 1) {
+      Expect.equals(4, e);
+      Expect.equals(3, d);
+      Expect.equals(8, c);
+      Expect.equals(1, b);
+      Expect.equals(32, a);
+    }
+    int f;
+    int k;
+    if (i < 20) {
+      f = b & c | ~b & d;
+      k = 0x5A827999;
+    } else if (i < 40) {
+      f = b ^ c ^ d;
+      k = 0x6ED9EBA1;
+    } else if (i < 60) {
+      f = b & c | b & d | c & d;
+      k = 0x8F1BBCDC;
+    } else {
+      f = b ^ c ^ d;
+      k = 0xCA62C1D6;
+    }
+
+    int temp = a << 5;
+    e = d;
+    d = c;
+    c = b << 2;
+    b = a;
+    a = temp;
+  }
+}
+