Make `cancel_inverses` also apply to gates through control flows

[paul0403]

Currently cancel_inverses does not detect potential opportunities if they are separated across the boundary of control flow constructs, e.g. for(4){Hadamard} or Hadamard, if(cond){True: Identity, False: Hadamard}. Being able to detect these opportunities would be a good addition.

This issue is more general than cancel_inverses. We essentially want to have an analysis available without having to explicitly unroll loops or hoist conditionals, since these could be expensive and are not guaranteed to happen.