v0.0.0-6162-ga6510014e

Improve verification performance by using a std:vector<bool> bitset

to keep track of seen ids, rather than an absl::flat_hash_set<Node*>.

40% relative improvement in time taken by VerifyNodeIdUnique for one workload
(2% of CPU time total down to 1.2%).

PiperOrigin-RevId: 686716610