dillon1000/react

Fix sloppy factoring in `performSyncWorkOnRoot` (#21246)

* Warn if `finishedLanes` is empty in commit phase

See #21233 for context.

* Fix sloppy factoring when assigning finishedLanes

`finishedLanes` is assigned in `performSyncWorkOnRoot` and
`performSyncWorkOnRoot`. It's meant to represent whichever lanes we
used to render, but because of some sloppy factoring, it can sometimes
equal `NoLanes`.

The fixes are:
- Always check if the lanes are not `NoLanes` before entering the work
loop. There was a branch where this wasn't always true.
- In `performSyncWorkOnRoot`, don't assume the next lanes are sync; the
priority may have changed, or they may have been flushed by a
previous task.
- Don't re-assign the `lanes` variable (the one that gets assigned to
`finishedLanes` until right before we enter the work loop, so that it
is always corresponds to the newest complete root.