[turbofan] Add test case for stores to properties that are also present on prototype.

test/mjsunit/compiler/regress-4470-1.js

Index: test/mjsunit/compiler/regress-4470-1.js
diff --git a/test/mjsunit/compiler/regress-491578.js b/test/mjsunit/compiler/regress-4470-1.js
similarity index 62%
copy from test/mjsunit/compiler/regress-491578.js
copy to test/mjsunit/compiler/regress-4470-1.js
index c27570456c31e9a191fe1f4f1ea17b80f29de98e..91d26b7212d4ef6a7413bf7d19dcfa350fdf87ca 100644
--- a/test/mjsunit/compiler/regress-491578.js
+++ b/test/mjsunit/compiler/regress-4470-1.js
@@ -4,12 +4,13 @@
 
 // Flags: --allow-natives-syntax
 
-function foo(x) {
-  if (x === undefined) return;
-  while (true) {
-    while (1 || 2) { }
-    f();
-  }
+function Foo() {}
+Foo.prototype.x = 0;
+function foo(f) {
+  f.x = 1;
 }
+foo(new Foo);
+foo(new Foo);
 %OptimizeFunctionOnNextCall(foo);
-foo();
+foo(new Foo);
+assertEquals(Foo.prototype.x, 0);