Bug fix in HoareOptimizer (backport #13083)

[mergify]

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 :).

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This is an automatic backport of pull request #13083 done by [Mergify](https://mergify.com).

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[coveralls]

Pull Request Test Coverage Report for Build 10705682221

Details

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• 45 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.05%) to 89.784%

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Files with Coverage Reduction	New Missed Lines	%
crates/circuit/src/dag_node.rs	6	81.5%
crates/qasm2/src/lex.rs	6	91.98%
crates/circuit/src/circuit_instruction.rs	9	88.51%
crates/qasm2/src/parse.rs	24	95.77%

Totals
Change from base Build 10687317978:	-0.05%
Covered Lines:	66882
Relevant Lines:	74492

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💛 - Coveralls