Option to toggle off compiler simplifications

Currently the compiler does several kinds of transformations, including mapping into a target gates set, routing onto a fixed qubit topology, and doing algebraic simplifications (e.g. X 0 \n X 0 is changed to an identity).

When doing things like benchmarking the chip, it can be useful to turn off the algebraic simplification. Otherwise, for example, all randomized benchmarking sequences will compile to an empty program.

A suggested interface could be to split the compiler to allow different passes in the flag, e.g.

backend.compile(pq, simplification=False)

Alternatively we could eventually expose different passes as different methods, e.g. backend.route(), backend.gate_transform, backend.simplify etc.

Start by tracing the backend.compile entry point and the compiler transformations described in the issue. Compare the proposed compile flag with separate pass methods, then verify that algebraic simplification can be disabled while gate mapping and topology routing still occur. Done means randomized benchmarking sequences no longer compile to an empty program solely because of simplification.