Qiskit / qiskit / 17941583232

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Commit Message

Fix edge-order non-determinism when adding DAG nodes (backport #15040) (#15057)

* Fix edge-order non-determinism when adding DAG nodes (#15040)

All of `DAGCircuit::push_back`, `DAGCircuit::push_front` and
`DAGCircuitBuilder::push_back` had non-determinisms in the precise order
that edges were added.  The two `DAGCircuit` methods had diverged, so
exhibited _different_ behaviour.  Most of the effects were limited to
nodes with multiple classical in edges, though `DAGCircuit::push_back`
did have some problems with high-degree quantum nodes (like barriers) as
well.

This commit unifies the code handled the two `DAGCircuit` methods to try
and prevent divergences (especially since `apply_operation_front` is
currently little used and little tested).  As a side effect, this fixed
a bug in `apply_operation_front` around handling multiple references to
the same clbit or var in classical expressions that was fixed in
`apply_operation_back` some time prior.

(cherry picked from commit 25c3206f0)

# Conflicts:
#	crates/circuit/src/dag_circuit.rs

* Fix merge conflict

---------

Co-authored-by: Jake Lishman <jake.lishman@ibm.com>
Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

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13 existing lines in 3 files now uncovered.

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