[turbofan] Fix lowering of Function.prototype accesses.

This fixes a corner case where the "instance prototype" diverges from
the "non-instance prototype" that we store on the initial map of a
constructor function.

R=bmeurer@chromium.org
TEST=mjsunit/regress/regress-crbug-703610
BUG=chromium:703610

Change-Id: I30a19ae621e10b512215ffb191ce00d030941440
Reviewed-on: https://chromium-review.googlesource.com/458396Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44008}

src/compiler/js-native-context-specialization.cc

@@ -846,7 +846,7 @@ Reduction JSNativeContextSpecialization::ReduceJSLoadNamed(Node* node) {
         // continue unless deoptimization is enabled.
         Handle<Map> initial_map(function->initial_map(), isolate());
         dependencies()->AssumeInitialMapCantChange(initial_map);
-        Handle<Object> prototype(initial_map->prototype(), isolate());
+        Handle<Object> prototype(function->prototype(), isolate());
         Node* value = jsgraph()->Constant(prototype);
         ReplaceWithValue(node, value);
         return Replace(value);

test/mjsunit/regress/regress-crbug-703610.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
+
+function fun() {};
+fun.prototype = 42;
+new fun();
+function f() {
+  return fun.prototype;
+}
+assertEquals(42, f());
+assertEquals(42, f());
+%OptimizeFunctionOnNextCall(f);
+assertEquals(42, f());