[turbofan] Optimize string "length" property access based on types.

test/mjsunit/compiler/string-length.js

+// Copyright 2015 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
+
+assertEquals(0, "".length);
+assertEquals(1, "a".length);
+assertEquals(2, ("a" + "b").length);
+
+function id(x) { return x; }
+
+function f1(x) {
+  return x.length;
+}
+assertEquals(0, f1(""));
+assertEquals(1, f1("a"));
+%OptimizeFunctionOnNextCall(f1);
+assertEquals(2, f1("a" + "b"));
+assertEquals(3, f1(id("a") + id("b" + id("c"))))
+
+function f2(x, y, z) {
+  x = x ? "" + y : "" + z;
+  return x.length;
+}
+assertEquals(0, f2(true, "", "a"));
+assertEquals(1, f2(false, "", "a"));
+%OptimizeFunctionOnNextCall(f2);
+assertEquals(0, f2(true, "", "a"));
+assertEquals(1, f2(false, "", "a"));
+assertEquals(3, f2(true, id("a") + id("b" + id("c")), ""));