Remove unecessary nodes field in SabreDAG (Qiskit#14199)

The SabreDAG struct was carrying a nodes field which was a Vec of the
topologically sorted node descriptions used to create the inner graph
object. The format of this field mirrors what Python passes to Rust when
initially creating the DAG. The reason this was being kept around was
for reversing the graph when working with control flow ops. However, in
practice this isn't necessary because all the data contained in the vec
is also contained in the graph and we can just reverse the graph
directly and save having to carry around the entire object. Also
removing this field simplifies the step of moving the sabre dag
construction into Rust which is required for porting the remainder of
the pass to Rust.

Author: mtreinish

crates/accelerate/src/sabre/layout.rs

@@ -243,21 +243,14 @@ fn layout_trial(
         num_qubits: dag.num_qubits,
         num_clbits: dag.num_clbits,
         dag: dag.dag.clone(),
-        nodes: dag.nodes.clone(),
         first_layer: dag.first_layer.clone(),
         node_blocks: dag
             .node_blocks
             .keys()
             .map(|index| (*index, Vec::new()))
             .collect(),
     };
-    let dag_no_control_reverse = SabreDAG::new(
-        dag_no_control_forward.num_qubits,
-        dag_no_control_forward.num_clbits,
-        dag_no_control_forward.nodes.iter().rev().cloned().collect(),
-        dag_no_control_forward.node_blocks.clone(),
-    )
-    .unwrap();
+    let dag_no_control_reverse = dag_no_control_forward.reverse_dag();
 
     for _iter in 0..max_iterations {
         for dag in [&dag_no_control_forward, &dag_no_control_reverse] {

crates/accelerate/src/sabre/sabre_dag.rs

@@ -42,10 +42,28 @@ pub struct SabreDAG {
     pub num_clbits: usize,
     pub dag: DiGraph<DAGNode, ()>,
     pub first_layer: Vec<NodeIndex>,
-    pub nodes: Vec<(usize, Vec<VirtualQubit>, HashSet<usize>, bool)>,
     pub node_blocks: HashMap<usize, Vec<SabreDAG>>,
 }
 
+impl SabreDAG {
+    pub fn reverse_dag(&self) -> Self {
+        let mut out_dag = self.clone();
+        out_dag.dag.reverse();
+        out_dag.first_layer = out_dag
+            .dag
+            .node_indices()
+            .filter(|idx| {
+                out_dag
+                    .dag
+                    .neighbors_directed(*idx, Incoming)
+                    .next()
+                    .is_none()
+            })
+            .collect();
+        out_dag
+    }
+}
+
 #[pymethods]
 impl SabreDAG {
     #[new]
@@ -105,7 +123,6 @@ impl SabreDAG {
             num_clbits,
             dag,
             first_layer,
-            nodes,
             node_blocks,
         })
     }