[regexp] Ensure IrregexpExecRaw is passed a flat string

test/mjsunit/regress/regress-5943.js

Index: test/mjsunit/regress/regress-5943.js
diff --git a/test/mjsunit/regress/regress-5943.js b/test/mjsunit/regress/regress-5943.js
new file mode 100644
index 0000000000000000000000000000000000000000..f51b1e9999b2188ed8e3acb0f65d5226eed52441
--- /dev/null
+++ b/test/mjsunit/regress/regress-5943.js
@@ -0,0 +1,14 @@
+// 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
+
+function createHTML() {
+  return '' + '<div><div><di';
+}
+
+createHTML();
+%OptimizeFunctionOnNextCall(createHTML);
+
+/./.test(createHTML());