Don't change interface for DAGCircuit

Previously this PR was making the op_names field of the DAGCircuit
struct public so it was accessible to the new transpiler pass code.
However, this opened up the possibility of mutating the field by
mistake, instead this makes the field private again and adds a no-copy
method to get an immutable reference to the field. Additionally, there
were some interface changes made to one method in DAGCircuit that were
not needed anymore, which this reverts to minimize the diff.

crates/accelerate/src/commutation_cancellation.rs

@@ -70,7 +70,7 @@ pub(crate) fn cancel_commutations(
         HashSet::new()
     };
     let z_var_gate = dag
-        .op_names
+        .get_op_counts()
         .keys()
         .find_map(|g| {
             VAR_Z_MAP

crates/circuit/src/dag_circuit.rs

@@ -269,7 +269,7 @@ pub struct DAGCircuit {
     var_output_map: _VarIndexMap,
 
     /// Operation kind to count
-    pub op_names: IndexMap<String, usize, RandomState>,
+    op_names: IndexMap<String, usize, RandomState>,
 
     // Python modules we need to frequently access (for now).
     control_flow_module: PyControlFlowModule,
@@ -6228,7 +6228,7 @@ impl DAGCircuit {
         &mut self,
         new_gate: (StandardGate, &[f64]),
         old_index: NodeIndex,
-    ) -> NodeIndex {
+    ) {
         self.increment_op(new_gate.0.name());
         let old_node = &self.dag[old_index];
         let inst = if let NodeType::Operation(old_node) = old_node {
@@ -6255,7 +6255,6 @@ impl DAGCircuit {
         self.dag.add_edge(parent_index, new_index, weight.clone());
         self.dag.add_edge(new_index, old_index, weight);
         self.dag.remove_edge(edge_index);
-        new_index
     }
 
     /// Remove a sequence of 1 qubit nodes from the dag
@@ -6455,6 +6454,16 @@ impl DAGCircuit {
         }
     }
 
+    /// Get an immutable reference to the op counts for this DAGCircuit
+    ///
+    /// This differs from count_ops() in that it doesn't handle control flow recursion at all
+    /// and it returns a reference instead of an owned copy. If you don't need to work with
+    /// control flow or ownership of the counts this is a more efficient alternative to
+    /// `DAGCircuit::count_ops(py, false)`
+    pub fn get_op_counts(&self) -> &IndexMap<String, usize, RandomState> {
+        &self.op_names
+    }
+
     /// Extends the DAG with valid instances of [PackedInstruction]
     pub fn extend<I>(&mut self, py: Python, iter: I) -> PyResult<Vec<NodeIndex>>
     where