Revert of [turbofan] Speculatively optimize string character access.

test/mjsunit/regress/regress-6391.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 --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);
-})();