Ensure scopes are backed by blocks in the body of for loops

test/mjsunit/regress/regress-536751.js

Index: test/mjsunit/regress/regress-536751.js
diff --git a/test/mjsunit/regress/regress-536751.js b/test/mjsunit/regress/regress-536751.js
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index 0000000000000000000000000000000000000000..b2d19e422a767ec2a20c0bf3a0f15dbe3d140762
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+// 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: --harmony-sloppy --harmony-sloppy-function --harmony-sloppy-let
+
+// At some point, this code led to DCHECK errors in debug mode
+
+for (; false;) function foo() {};
+
+for (x in []) function foo() {};