Add conversion transpiler pass from c_if to IfElseOp

[jakelishman]

Summary

This is intended to be used to ease the transition from the old-style conditions to the new-style form. Backends may use it in their custom pipelines, so they can guarantee they only need to deal with one form.

A further classical optimisation may be to group IfElseOp blocks that share the same condition into a single operation, but this would likely be a separate pass, if added, or potentially be the domain of a separate part of the circuit-description/compilation/running pipeline.

Details

Fixes #8601.

Will need rebasing over #8627 and #8615 once they merge, as this depends on them, and currently includes their commits. I just can't look at my screen any more tonight to wait til it merges.

edit: This is now rebased.

cc: @taalexander (this is the pass I wrote for you)

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[coveralls]

Pull Request Test Coverage Report for Build 3147692364

• 47 of 47 (100.0%) changed or added relevant lines in 3 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage remained the same at 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 3147690270:	0%
Covered Lines:	60254
Relevant Lines:	71305

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💛 - Coveralls

[mtreinish]

Overall the code LGTM, this is a pretty straightforward replacement and you're more thorough than I would have been as I never would have thought about a user using c_if inside a control flow block (but that could be because I'm not really used to dealing with recursive pass calling yet to work with control flow).

The only thing that I think would be good before we merge is some negative (as in not conditional instructions) test coverage. I think just a couple test to test a dag with no conditional operations to validate that it is unchanged on the output and also a test checking a control flow block with no conditional operations inside it (which was my one inline comment). They're not super important but just asserting this as part of the api will hopefully prevent bugs in the future just in case.