[regexp] Properly flatten string during initialization

test/mjsunit/regress/regress-698790.js

+// 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);