[DomTree] Reduce number of hash table lookups (NFC) (#73097)

Inside runSemiNCA(), create a direct mapping from node number of node
info, so we can save the node number -> node pointer -> node info lookup
in many cases.

To allow this in more cases, change Label to a node number instead of
node pointer.

I've limited this to runSemiNCA() for now, because we have the
convenient property there that no new node infos will be added, so we
don't have to worry about pointer invalidation.

This gives a pretty nice compile-time improvement of about 0.4%.