What should we add?

Part of #9417. Depends on:

• Support control flow in SabreSwap #9419
• Support control flow in Python components of SabreLayout #9421

In #8830 we had to revert optimisation level 1 to use its legacy behaviour of laying out with DenseLayout and routing with StochasticSwap if the circuit was a control-flow circuit, since the new default SabreSwap couldn't handle the control-flow. Once SabreSwap supports control-flow, we should remove this switching logic, and swap back to the preferred router.

SabreLayout should be easier to upgrade to initial support of control-flow (especially since it'll technically already run, it'll just not have good output), if only the Python-space paths are upgraded.