Avoid deepcopy in pulse parser

[jakelishman]

Summary

This deepcopy is not strictly necessary; since we already own the nodes we're modifying, we only need to shallow copy nodes that we want to replace. In practice, the deep copy seems to cause difficult-to-resolve reference cycles within Jupyter that can get the parser.

Details and comments

Fix #8783.

I was never able to use any of the cells given in that issue to reliably reproduce the issue, but I did seem to hit a similar issue when trying to transpile for an internal backend. This just ensures that all nodes that need to be mutated during the expression visit are only shallow copied, avoiding the deep-copy problem. I don't understand why that caused particular issues, though.

@nkanazawa1989: perhaps you can check me here, but I believe this should be fine and safe.

[qiskit-bot]

Thank you for opening a new pull request.

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.

One or more of the the following people are requested to review this:

• @Qiskit/terra-core
• @nkanazawa1989

[mtreinish]

This LGTM, but I'm going to hold off on tagging this as automerge until @nkanazawa1989 can take a look because he's the most familiar with this code.

[coveralls]

Pull Request Test Coverage Report for Build 3384677280

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• 115 unchanged lines in 6 files lost coverage.
• Overall coverage decreased (-0.2%) to 84.359%

---

Files with Coverage Reduction	New Missed Lines	%
src/optimize_1q_gates.rs	1	95.16%
qiskit/extensions/quantum_initializer/squ.py	2	79.78%
qiskit/pulse/library/waveform.py	3	91.49%
src/sampled_exp_val.rs	14	63.93%
src/results/marginalization.rs	36	67.82%
src/sabre_swap/mod.rs	59	80.94%

Totals
Change from base Build 3381988739:	-0.2%
Covered Lines:	62255
Relevant Lines:	73798

---

💛 - Coveralls

[nkanazawa1989]

I wrote this almost 3 years ago but I don't quite remember why I needed the deep copy of the AST node here. This copy occurs to support parameter partial binding, so I believe this change should be okey as long as unittest for partial bindings works normally. Probably we can remove copy as well. Anyways partial binding will never occur in the current use case, and indeed this parser is sort of overengineering. Please go ahead with merging.