[interpreter] Make sure allocated registers are always materialized in the register optimizer.

test/mjsunit/compiler/regress-729369.js

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax
+
+function* f() {
+  x.__defineGetter__();
+  var x = 0;
+  for (let y of iterable) {
+    yield y;
+  }
+}
+
+f();