[turbofan] Mark loads of a module (from a module context) immutable.

test/mjsunit/modules-turbo2.js

Index: test/mjsunit/modules-turbo2.js
diff --git a/test/mjsunit/modules-turbo2.js b/test/mjsunit/modules-turbo2.js
new file mode 100644
index 0000000000000000000000000000000000000000..734b2c409323945449e2f86759f138d1f5098f95
--- /dev/null
+++ b/test/mjsunit/modules-turbo2.js
@@ -0,0 +1,17 @@
+// 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.
+
+// MODULE
+// Flags: --allow-natives-syntax --turbo
+
+export let x = 0;
+
+function foo() { return x++ };
+
+function gaga(f) { return f() };
+
+assertEquals(0, gaga(foo));
+assertEquals(1, gaga(foo));
+%OptimizeFunctionOnNextCall(gaga);
+assertEquals(2, gaga(foo));