[turbofan] Teach escape analysis about StringCharAt

test/mjsunit/regress/regress-crbug-677757.js

Index: test/mjsunit/regress/regress-crbug-677757.js
diff --git a/test/mjsunit/regress/regress-crbug-677757.js b/test/mjsunit/regress/regress-crbug-677757.js
new file mode 100644
index 0000000000000000000000000000000000000000..044970c27b3a09e8a9562934e7a88583beefb07f
--- /dev/null
+++ b/test/mjsunit/regress/regress-crbug-677757.js
@@ -0,0 +1,7 @@
+// 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.
+
+for (var i = 0; i < 50000; i++) {
+ ("Foo"[0] + "barbarbarbarbarbar")[0];
+}