Make heuristic score sorting robust in 1q optimization pass

In Qiskit#8197 the Optimize1qGatesDecomposition was updated to be target aware
and as part of that a new heuristic scoring was added to determine which
decomposition was the best performing and should be used. However, in
the case of an error rate of 0.0 the desired behavior was to pick the
decomposition which was shortest. In Qiskit#8197 this was accomplished by just
doing `-100 + len(decomposition)` which worked in the general case as
long as the gate counts are < 100 (which in practice will always be the
case). For robustness this changes the return for the scoring function
to be `(error_rate, sequence_length)` to give us the sorting without
changing the measured error rate.

qiskit/transpiler/passes/optimization/optimize_1q_decomposition.py

@@ -112,13 +112,18 @@ def _substitution_checks(self, dag, old_run, new_circ, basis, qubit):
         # if we're outside of the basis set, we're obligated to logically decompose.
         # if we're outside of the set of gates for which we have physical definitions,
         #    then we _try_ to decompose, using the results if we see improvement.
+        new_error = _error(new_circ, self._target, qubit)
+        if isinstance(new_error, tuple):
+            error_rate = new_error[0]
+        else:
+            error_rate = new_error
         return (
             uncalibrated_and_not_basis_p
             or (
                 uncalibrated_p
-                and _error(new_circ, self._target, qubit) < _error(old_run, self._target, qubit)
+                and new_error < _error(old_run, self._target, qubit)
             )
-            or np.isclose(_error(new_circ, self._target, qubit), 0)
+            or np.isclose(error_rate, 0)
         )
 
     @control_flow.trivial_recurse
@@ -189,7 +194,4 @@ def _error(circuit, target, qubit):
                 for inst in circuit
             ]
         gate_error = 1 - np.product(gate_fidelities)
-        if gate_error == 0.0:
-            return -100 + len(circuit)  # prefer shorter circuits among those with zero error
-        else:
-            return gate_error
+        return (gate_error, len(circuit))