Fix control-flow routing in StochasticSwap

[jakelishman]

Summary

The StochasticSwap pass has some fairly complex hand-offs between
different parts of its API, including in recursive calls and the regular
hand-off between Python and Rust. In the course of adding the
control-flow support, some of these became muddled, and the mapping
between different virtual/physical/integer representations got mixed up,
resulting in invalid swaps being output in the final circuit.

This commit simplifies much of the internal mapping, removing many
superfluous DAGCircuit creations and compositions. This also removes
instances where two layouts were "chained"; this was not well typed (the
output of a "virtual -> physical" mapping can't be the input for another
"virtual -> physical" mapping), and in general was being used to "undo"
some of compositions that were about to be applied.

This fixes a tacit assumption in the original code that the initial
layout was a trivial layout in the hand-off between Rust and Python.
This worked until the recursive call added the initial_layout option,
making this assumption invalid. Previously, virtual qubit bit instances
were converted to integers (to allow them to be passed to Rust) using
their indices into the original DAG, but the integer outputs were then
converted back using the initial_layout. In the old form, this
worked anyway, but wasn't logically correct and consequently broke when
the assumptions about initial_layout changed.

For the recursive calls, we now ensure that the inner passes are
essentially created with the same internal structure as the outer pass;
the passed in DAGCircuit uses the same bit instances and same meaning
of the virtual qubits as the outer circuit, and the initial_layout
ensures that the inner passes start with at the same layout as the outer
pass. This makes the inner passes more like a logical continuation of
the current operation, rather than a completely separate entity that
needs to have its virtual qubits remapped.

The changes to the tests are twofold:

• move the CheckMap calls earlier and apply them directly to the
  StochasticSwap output rather than the expected circuit, to improve
  the quality of failure error messages

• use the same physical qubits inside the expected control-flow blocks;
  the new simpler form of doing the circuit rewriting internally in the
  pass ensures that the same bit objects are used all the way through
  the control-flow stack now, rather than creating new instances.

Details and comments

Also has a patch from Matthew to add a bunch more random-circuit tests. Tests need #8879 to merge before they'll pass due to a bug in Unroll3qOrMore.

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[mtreinish]

Overall this LGTM! It's a lot easier to follow the layout and transforms being performed now. Also the updated tests give us a bit more confidence the output is valid. The only thing I think is missing from our test path is that we're generating an equivalent output, but we can save this for the randomized testing I think (by adding control flow to the tests there), since I tried to do that in the new test class here and it was prohibitively expensive for the unit tests.

[mtreinish]

Heh, this was a useful

[mtreinish]

(not tagging as automerge until this is rebased on #8879 which is needed for the tests to pass)

[ewinston]

Thanks for tracking this bug and simplifying the code further!

[coveralls]

Pull Request Test Coverage Report for Build 3237543802

• 32 of 34 (94.12%) changed or added relevant lines in 2 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.008%) to 84.755%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/transpiler/passes/routing/stochastic_swap.py	28	30	93.33%

Files with Coverage Reduction	New Missed Lines	%
qiskit/transpiler/passes/routing/utils.py	1	84.62%
qiskit/extensions/quantum_initializer/squ.py	2	79.78%

Totals
Change from base Build 3237541004:	-0.008%
Covered Lines:	61884
Relevant Lines:	73015

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💛 - Coveralls