Restrict `Split2QUnitaries` to run on `UnitaryGate`s only (backport #13095)

[mergify[bot]]

Summary

Fixes the remaining problems with #12970.

Details and comments

If we run Split2QUnitaries with an incomplete 1-q basis gate set, we can end up in the situation where a supported gate is split into 2 UnitaryGate objects that might not be representable in the current basis gate set. For example

from qiskit import QuantumCircuit, transpile
from qiskit.circuit.library import *

gate = PauliGate("XX")

qc = QuantumCircuit(2)
qc.append(gate, [0, 1])

basis_gates = ["x"]  # gate decomposes into XX

circuit = transpile(qc, basis_gates=basis_gates, optimization_level=2)  # fails -- cannot map U to the basis

This PR fixes the behavior by only running on UnitaryGate objects -- hence not changing the gate types existing in the circuit.

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This is an automatic backport of pull request #13095 done by Mergify.

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[coveralls]

Pull Request Test Coverage Report for Build 10725172384

Details

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• 19 unchanged lines in 6 files lost coverage.
• Overall coverage increased (+0.005%) to 89.779%

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Files with Coverage Reduction	New Missed Lines	%
crates/circuit/src/packed_instruction.rs	1	96.21%
crates/accelerate/src/results/marginalization.rs	1	90.45%
crates/accelerate/src/synthesis/clifford/utils.rs	1	78.06%
crates/qasm2/src/lex.rs	2	92.98%
crates/circuit/src/circuit_instruction.rs	7	86.33%
crates/circuit/src/operations.rs	7	88.31%
<!--	Total:	19		-->

Totals
Change from base Build 10719167310:	0.005%
Covered Lines:	66861
Relevant Lines:	74473

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💛 - Coveralls