Fix QuantumCircuit.decompose for high-level objects

[Cryoris]

Summary

Fixed some problems with QuantumCircuit.decompose when dealing with high-level objects (i.e. that can get synthesized by HighLevelSynthesis).

Blocks #13295 and #13301.

Details and comments

Previously, running decompose would first run HighLevelSynthesis (in case no specific gates to decompose are set) and the Decompose. This would end up decomposing high-level objects twice instead of once. In addition, specifying high-level objects by name, could end up not decomposing them at all.

For example

Circuit at hand:
   ┌──────────┐
q: ┤ Clifford ├
   └──────────┘
Expected result when decomposing:
   ┌───┐
q: ┤ H ├
   └───┘
.decompose(['clifford']) gives:
   ┌──────────┐
q: ┤ Clifford ├
   └──────────┘
.decompose() gives:
   ┌─────────┐
q: ┤ U2(0,π) ├
   └─────────┘
where's my H gate? :(

This is now fixed by pulling HLS into Decompose, which iterates over each node and only applies HLS if an operation does not have a definition.

[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 11288875324

Details

• 23 of 23 (100.0%) changed or added relevant lines in 2 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.01%) to 88.666%

---

Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	3	92.73%

Totals
Change from base Build 11277520565:	0.01%
Covered Lines:	73138
Relevant Lines:	82487

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💛 - Coveralls

[alexanderivrii]

Thanks @Cryoris for making these fixes!