[turbofan] Disable usage of {maybe_assigned} variable flag.

test/mjsunit/regress/regress-crbug-659915a.js

+// Copyright 2016 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 --min-preparse-length=10
+
+let x;
+function f(a) {
+  x += a;
+}
+function g(a) {
+  f(a); return x;
+}
+function h(a) {
+  x = a; return x;
+}
+
+function boom() { return g(1) }
+
+assertEquals(1, h(1));
+assertEquals(2, boom());
+assertEquals(3, boom());
+%OptimizeFunctionOnNextCall(boom);
+assertEquals(4, boom());