Avoid iterating over nodes in Split2QUnitaries if there are no unitaries

[mtreinish]

Summary

This commit adds a check to the start of the split_2q_unitaries rust function that is the core of the Split2QUnitaries transpiler pass to do a lookup if there are any UnitaryGate objects in the dag without iterating over the dag. The pass only will split unitary gates so we can avoid checking the name of each gate individually if we know there aren't any up front.

Details and comments

[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 11601844816

Details

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 88.741%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	2	93.73%

Totals
Change from base Build 11601286937:	0.02%
Covered Lines:	75700
Relevant Lines:	85304

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💛 - Coveralls

[Cryoris]

Getting the op counts also iterates over the DAG though right? Would we get a similar or even better performance if we have the name-check before we get the qubits? I.e.

for node in nodes {
   if let NodeType::Operation(inst) = &dag.dag()[node] { 
       if inst.op.name() != "unitary" { continue };
       // rest as before ...

This way we would keep only one loop over the DAG, no?

[mtreinish]

No, it's an O(1) lookup that doesn't require any iteration. The DAGCircuit maintains the counts mapping as operations are added and removed specifically for this use case. You can just look at the method definition: https://github.com/Qiskit/qiskit/blob/main/crates/circuit/src/dag_circuit.rs#L6634-L6642