[turbofan] Speculatively optimize string character access.

test/mjsunit/regress/regress-6391.js

Index: test/mjsunit/regress/regress-6391.js
diff --git a/test/mjsunit/regress/regress-6391.js b/test/mjsunit/regress/regress-6391.js
new file mode 100644
index 0000000000000000000000000000000000000000..db0cef00059653c57e568e66734bc8e81e6b43c7
--- /dev/null
+++ b/test/mjsunit/regress/regress-6391.js
@@ -0,0 +1,27 @@
+// 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 --opt --no-always-opt
+
+(function() {
+  function foo(i) { return "a".charAt(i); }
+
+  assertEquals("a", foo(0));
+  assertEquals("a", foo(0));
+  assertEquals("", foo(1));
+  %OptimizeFunctionOnNextCall(foo);
+  assertEquals("", foo(1));
+  assertOptimized(foo);
+})();
+
+(function() {
+  function foo(i) { return "a".charCodeAt(i); }
+
+  assertEquals(97, foo(0));
+  assertEquals(97, foo(0));
+  assertEquals(NaN, foo(1));
+  %OptimizeFunctionOnNextCall(foo);
+  assertEquals(NaN, foo(1));
+  assertOptimized(foo);
+})();