Assertion failure in InlineCache.cpp

I discovered this while trying to produce a repro for #6160. The following code when run in `ch` hits an assertion in `InlineCache.cpp`:

```js
WScript.Attach(() => {
let iter = g();
iter.next();
iter.next();
});
(async () => {
await 777;
await 1206;
WScript.Detach(() => {});
})();

function* g() { yield 0; }
```

The assertion that was hit is:
https://github.com/microsoft/ChakraCore/blob/master/lib/Runtime/Language/InlineCache.cpp#L570
```cpp
Assert(!needsProtoInvalidation || isProtoRegistered);
```