Bug fix in HoareOptimizer

[alexanderivrii]

Summary

Fixes #13079.

Detailed Comments

The problem was due to first simplifying a controlled-gate by removing its controls (in the example: the control qubit of a CX-gate is at |1>, allowing to replace the CX-gate by an X-gate), then incorrectly keeping track of the simplified gate (in order to store the new X-gate for later purposes, the code mutated the original CX-gate in some weird way).

The code was already using substitute_node_with_dag to replace the controlled-gate by the base-gate (I don't really like this, but I don't think we can use substitute_node as the simplified node has fewer qubits than the original node). To fix the problem, we can instead read off the simplified gate from substitute_node_with_dag's output.

While this fixes the immediate problem, the code in HoareOptimizer is very messy and quite possibly contains many other sources of bugs (for instance, I did not like the idea of iterating over the DAG's topological nodes, while removing and simplifying some of the nodes in-place) and not optimized for performance. A complete rewrite while porting to Rust would be ideal :).

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[coveralls]

Pull Request Test Coverage Report for Build 10701833297

Details

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• 32 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.03%) to 89.159%

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Files with Coverage Reduction	New Missed Lines	%
qiskit/synthesis/two_qubit/xx_decompose/decomposer.py	1	95.42%
crates/qasm2/src/lex.rs	2	92.23%
crates/circuit/src/circuit_instruction.rs	5	86.32%
crates/circuit/src/dag_node.rs	6	82.17%
crates/qasm2/src/parse.rs	18	96.69%

Totals
Change from base Build 10691830429:	-0.03%
Covered Lines:	72513
Relevant Lines:	81330

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💛 - Coveralls