Avoid routing 2q barriers in SabreSwap

[mtreinish]

Summary

This commit fixes an oversight in the sabre swap pass where a 2 qubit barrier would have been treated like a 2 qubit gate and swaps could potentially be inserted if the sabre algorithm thought it didn't have connectivity for the qargs to the barrier. However as barrier is just a compiler directive it is valid on all qubit pairs so this swap insertion was unnecessary, it was still valid but just not an optimal output. This commit fixes it by adding context to the sabre dag around whether a given node is a directive (and valid on all qubits) or not.

Details and comments

[qiskit-bot]

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

• @Eric-Arellano
• @Qiskit/terra-core
• @kevinhartman
• @mtreinish

[coveralls]

Pull Request Test Coverage Report for Build 6962479250

• 36 of 36 (100.0%) changed or added relevant lines in 2 files are covered.
• 11 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.01%) to 86.425%

---

Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	5	90.91%
crates/qasm2/src/parse.rs	6	96.2%

Totals
Change from base Build 6961554254:	-0.01%
Covered Lines:	66273
Relevant Lines:	76683

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💛 - Coveralls

[kevinhartman]

I believe you also need to update extended set population to ignore directives:

if required_predecessors[successor_index] == 0 {
    if !dag.dag[node].directive && !dag.node_blocks.contains_key(&successor_index) {
        if let [a, b] = dag.dag[successor_node].qubits[..] {
            extended_set.push([a.to_phys(layout), b.to_phys(layout)]);
        }
    }
    to_visit.push(successor_node);
}

Otherwise, this looks good!

[mtreinish]

I believe you also need to update extended set population to ignore directives:

Done in: 89da56e

[kevinhartman]

LGTM, thanks for the updates!