add control flow support to UnitarySynthesis pass

[ewinston]

Summary

Add control flow support to UnitarySynthesis transpiler pass.

Details and comments

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[coveralls]

Pull Request Test Coverage Report for Build 3148714516

• 9 of 9 (100.0%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 84.502%

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Files with Coverage Reduction	New Missed Lines	%
qiskit/extensions/quantum_initializer/squ.py	2	79.78%

Totals
Change from base Build 3148712843:	0.2%
Covered Lines:	60261
Relevant Lines:	71313

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💛 - Coveralls

[jakelishman]

From the meeting: we might want to replace this with the utility decorator on top of the run method to do the trivial recursion.

[jakelishman]

I thought I'd added this further comment here already, sorry:

I think this is one pass we'll want to do a little bit of a refactor before doing the recursion - it does a lot of non-trivial setup work before getting to the core, and we'll want to make sure we don't repeat that every time. It should still be easily doable with the helper in #8752, but it probably wants the inner core refactoring into "setup", "do", and "teardown" components, so we can only recurse the "do" part.

[jakelishman]

As discussed, I've updated this PR to only recurse the post-setup "run" loop component of the pass, and to use the new utility functions added in #8752.