Apply Peephole Optimization Before Routing

[sbrandhsn]

What should we add?

We should investigate to apply peephole optimization before routing.

[mtreinish]

In principle I agree this is a really good idea, and now that 2q unitary synthesis is quite fast we can get a lot of potential benefit out of this. However, we have tried this in the past for 1q in #11354 but had to revert it in #11360 because it caused issues for people targeting discrete basis sets. The trick for this will be around making sure we don't break that use case in adding this. We'll probably want some conditions in the pass manager construction to avoid doing this in certain conditions.