add control flow handling to DenseLayout pass

[ewinston]

Summary

Add control flow handling to dense layout pass.

Details and comments

depends on #8763

todo: may still need to update Target.operation_names_for_qargs when target object is supplied.

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[coveralls]

Pull Request Test Coverage Report for Build 3156699536

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.05%) to 84.646%

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Files with Coverage Reduction	New Missed Lines	%
qiskit/pulse/library/waveform.py	3	91.49%

Totals
Change from base Build 3156697731:	0.05%
Covered Lines:	61419
Relevant Lines:	72560

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💛 - Coveralls

[mtreinish]

FWIW, I'm not sure how much this is actually needed. The cx count and measurement count are only used as weighting factors when trying to estimate how noisy the qubits in a layout are (in the BackendV1 path only). For control flow circuits trying to adjust that count really doesn't provide much benefit over not doing it because it's just a heuristic to try and avoid super noisy qubits or 2q links being selected in the layout (the weighting factor is just to care more about 2q error vs measurements based on the circuit which is why we excluded it from the v2 path). This error based heuristic is kind of obsolete now anyway with VF2PostLayout which does a much better job at noise aware qubit selection post routing so we probably could remove the heuristic from the pass now.

[jakelishman]

#8763 will automatically upgrade DenseLayout to recurse into control-flow when counting up the cx and measure gates, and that PR is perhaps a bit more logically the right location for such a change. I get the argument that DenseLayout's heuristics are going to be a bit rubbish for control flow, but I think having the explicit test, and just having them be automatically upgraded is fine - if we want to revisit the heuristic, it's better to do that in a non-control-flow specific PR anyway.

[jakelishman]

I've rebased this properly over #8763 now that it's merged. The principal remaining change is now mostly the test, and since this is just a simple pass, I think this is fine.

[jakelishman]

The heuristic here is really pretty meaningless (and clearly intended for backends that have cx as their only 2+q gate), and the Target path doesn't even attempt to put a number on it. I'm not bothered at all about doing more work on the heuristic, since it's already not really doing anything.