[verifier] [bug] Fix a bug when removing common prefix gates and do not generate hashing distribution in test_optimization_steps.h

[xumingkuan]

This PR fixes a bug in Verifier::equivalent when removing common prefix gates, that if the first circuit is a prefix of the second circuit, this function would also remove the entire second circuit before this PR. After this PR, this function will correctly keep the remaining part of the second circuit.

This PR also avoids generating the hashing distribution in test_optimization_steps.h. The hashing distribution is only for CircuitSeq::hash(), and it takes up space of Theta(2^num_qubits). This size is huge when the number of qubits is greater than 30. We can test circuits with more than 30 qubits after this PR.

Also adds some debugging output when verbose is true.