What should we add?

This pass would be very useful to ensure that negligible gates are not needlessly applied. In Cirq, whether a gate is negligible is determined by its implementation of the trace distance bound protocol: see https://github.com/quantumlib/Cirq/blob/3e3d6f7d9368badeafd0f2ca3ee0929ab0fdaa25/cirq-core/cirq/transformers/drop_negligible_operations.py. I'm not sure yet what would be the best way to implement this in Terra.