[turbofan] Don't merge PropertyAccessInfos with different field maps.

test/mjsunit/regress/regress-crbug-679378.js

Index: test/mjsunit/regress/regress-crbug-679378.js
diff --git a/test/mjsunit/regress/regress-crbug-679378.js b/test/mjsunit/regress/regress-crbug-679378.js
new file mode 100644
index 0000000000000000000000000000000000000000..fded13fc81543936c4d28c29a591b242064da322
--- /dev/null
+++ b/test/mjsunit/regress/regress-crbug-679378.js
@@ -0,0 +1,19 @@
+// 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
+
+var x = {};
+x.__defineGetter__('0', () => 0);
+x.a = {v: 1.51};
+
+var y = {};
+y.a = {u:"OK"};
+
+function foo(o) { return o.a.u; }
+foo(y);
+foo(y);
+foo(x);
+%OptimizeFunctionOnNextCall(foo);
+%DebugPrint(foo(x));