Avoid unnecessary matrix construction in Split2qUnitaries (#13378)

During the transition of this pass to Rust, the matrix creation was
accidentally moved before the check of whether the pass would operate on
the node.  This wastes time and allocations on creating matrices that
will not be further examined.

crates/accelerate/src/split_2q_unitaries.rs

@@ -33,15 +33,18 @@ pub fn split_2q_unitaries(
     for node in nodes {
         if let NodeType::Operation(inst) = &dag.dag()[node] {
             let qubits = dag.get_qargs(inst.qubits).to_vec();
-            let matrix = inst.op.matrix(inst.params_view());
             // We only attempt to split UnitaryGate objects, but this could be extended in future
             // -- however we need to ensure that we can compile the resulting single-qubit unitaries
             // to the supported basis gate set.
             if qubits.len() != 2 || inst.op.name() != "unitary" {
                 continue;
             }
+            let matrix = inst
+                .op
+                .matrix(inst.params_view())
+                .expect("'unitary' gates should always have a matrix form");
             let decomp = TwoQubitWeylDecomposition::new_inner(
-                matrix.unwrap().view(),
+                matrix.view(),
                 Some(requested_fidelity),
                 None,
             )?;