[regexp] Properly flatten string during initialization

test/mjsunit/regress/regress-698790.js

Index: test/mjsunit/regress/regress-698790.js
diff --git a/test/mjsunit/regress/regress-698790.js b/test/mjsunit/regress/regress-698790.js
new file mode 100644
index 0000000000000000000000000000000000000000..e20a77189a47b0704e8e6819bd8a3083597907d9
--- /dev/null
+++ b/test/mjsunit/regress/regress-698790.js
@@ -0,0 +1,18 @@
+// 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 --turbo
+
+// Call RegExp constructor with a cons string.
+
+var cons_string = %ConstructConsString("", "aaaaaaaaaaaaaa");
+new RegExp(cons_string);
+
+// Same thing but using TF lowering.
+
+function make_cons_string(s) { return s + "aaaaaaaaaaaaaa"; }
+make_cons_string("");
+%OptimizeFunctionOnNextCall(make_cons_string);
+var cons_str = make_cons_string("");
+new RegExp(cons_str);