perf(Subscription): use instanceof to avoid megamorphic LoadIC

In `Subscription#add()` use `instanceof` to test whether `subscription`
must be wrapped in a `Subscription` (aka for presence of `_addParent`
method), instead of `typeof subscription._addParent === 'function'`,
as the latter is going to turn into a *megamorphic property access*
in any realistic application and cause quite a bit of contention on
the megamorphic stub cache in Node (aka V8).

The `instanceof` is definitely faster, even if the property access
would hit the megamorphic stub cache.

src/internal/Subscription.ts

@@ -154,7 +154,7 @@ export class Subscription implements SubscriptionLike {
         } else if (this.closed) {
           subscription.unsubscribe();
           return subscription;
-        } else if (typeof subscription._addParent !== 'function' /* quack quack */) {
+        } else if (!(subscription instanceof Subscription)) {
           const tmp = subscription;
           subscription = new Subscription();
           subscription._subscriptions = [tmp];