refactor(combineLatest): small efficiency changes (#6021)
refactor(combineLatest): small efficiency changes (#6021)

- no longer use an array to track first value emission
- inline a few things than can be inlined
- adds more comments
martinsik/rxjs,ReactiveX/rxjs,ReactiveX/rxjs,benlesh/RxJS,benlesh/RxJS,ReactiveX/RxJS,ReactiveX/RxJS,martinsik/rxjs,benlesh/RxJS,benlesh/RxJS,ReactiveX/rxjs,ReactiveX/RxJS,martinsik/rxjs,ReactiveX/rxjs,martinsik/rxjs,ReactiveX/RxJS