[compiler] Improve let+const decision in AstNumbering.

test/mjsunit/regress/regress-v8-5666.js

Index: test/mjsunit/regress/regress-v8-5666.js
diff --git a/test/mjsunit/regress/regress-v8-5666.js b/test/mjsunit/regress/regress-v8-5666.js
new file mode 100644
index 0000000000000000000000000000000000000000..a023a9ae18365f3ce5c6cbd88eb9dbcc0d9d32f9
--- /dev/null
+++ b/test/mjsunit/regress/regress-v8-5666.js
@@ -0,0 +1,18 @@
+// Copyright 2016 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 foo(j) {
+  for (var i = 0; i < j + 1; i++) {
+    if (i === j) %OptimizeOsr();
+  }
+  let k = 1
+}
+
+foo(1000);
+foo(1000);
+%OptimizeFunctionOnNextCall(foo);
+foo(-1);
+assertOptimized(foo);