Fixed bug in OptimizeSwapBeforeMeasure

[border-b]

Summary

Fixed an issue with the :class:~.OptimizeSwapBeforeMeasure pass where it would incorrectly optimize circuits involving swap and measure instructions. This commit fixes the bug by changingDAGCircuit.successorstoDAGCircuit.descendants. Also, added a couple of extra tests to ensure that the bug is fixed.

Details and comments

The original issue in #11195 could not be reproduced after the fix as it produced a ValueError.

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Before your PR can be merged it will first need to pass continuous integration tests and be reviewed. Sometimes the review process can be slow, so please be patient.

While you're waiting, please feel free to review other open PRs. While only a subset of people are authorized to approve pull requests for merging, everyone is encouraged to review open pull requests. Doing reviews helps reduce the burden on the core team and helps make the project's code better for everyone.

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[coveralls]

Pull Request Test Coverage Report for Build 7208145798

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 11 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.002%) to 87.536%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	5	91.41%
crates/qasm2/src/parse.rs	6	96.67%

Totals
Change from base Build 7203025354:	-0.002%
Covered Lines:	59761
Relevant Lines:	68270

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💛 - Coveralls

[jakelishman]

This is excellent, thank you very much! Great, clear tests.