[turbofan] Fix bogus materialization from frame with OSR.

test/mjsunit/regress/regress-osr-context.js

+// Copyright 2015 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 --context-specialization --turbo-filter=f
+
+(function() {
+  "use strict";
+  var a = 23;
+  function f() {
+    for (let i = 0; i < 5; ++i) {
+      a--;  // Make sure {a} is non-immutable, hence context allocated.
+      function g() { return i }  // Make sure block has a context.
+      if (i == 2) %OptimizeOsr();
+    }
+    return a;
+  }
+  assertEquals(18, f());
+})();