Cleanup: Move mjsunit/regress-*.js into mjsunit/regress/.

test/mjsunit/regress-3456.js

-// Copyright 2014 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.
-
-// Arrow function parsing (commit r22366) changed the flags stored in
-// PreParserExpression, and IsValidReferenceExpression() would return
-// false for certain valid expressions. This case is the minimum amount
-// of code needed to validate that IsValidReferenceExpression() works
-// properly. If it does not, a ReferenceError is thrown during parsing.
-
-function f() { ++(this.foo) }