[crankshaft] Fix string addition to check for max length of cons string.

test/mjsunit/regress/regress-678917.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.
+
+s1 = 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx';
+s1 += s1;
+s1 += s1;
+s1 += s1;
+s1 += s1;
+
+s0 = 'a';
+
+function g() {
+  for (var j = 0; j < 1000000; j++) {
+    s0 += s1;
+  }
+}
+
+try {
+  g();
+} catch (e) {
+}
+
+assertEquals('x', s0[10]);