[DomTree] Store ReverseChildren as indices (NFC) (#73505)

Store the ReverseChildren using node indices instead of node pointers.
This avoids some more hash table lookups.

I've also increased the size of the SmallVector from 2 to 4. As the
indices are half as large as the pointers (on 64bit) this keeps memory
usage the same as before. I've found the larger SmallVector to perform a
bit better.