dillon1000/react

[DevTools] Reconcile Fibers Against Previous Children Instances (#30822)

This loops over the remainingReconcilingChildren to find existing
FiberInstances that match the updated Fiber. This is the same thing we
already do for virtual instances. This avoids the need for a
`fiberToFiberInstanceMap`.

This loop is fast but there is a downside when the children set is very
large and gets reordered with keys since we might have to loop over the
set multiple times to get to the instances in the bottom. If that
becomes a problem we can optimize it the same way ReactChildFiber does
which is to create a temporary Map only when the children don't line up
properly. That way everything except the first pass can use the Map but
there's no need to create it eagerly.

Now that we have the loop we don't need the previousSibling field so we
can save some memory there.