nak: fix clippy::manual_while_let_some warnings

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27234>

src/nouveau/compiler/nak/assign_regs.rs

@@ -712,8 +712,7 @@ fn instr_assign_regs_file(
     let mut vec_dsts_map_to_killed_srcs = true;
     let mut could_trivially_allocate = true;
     for vec_dst in vec_dsts.iter_mut().rev() {
-        while !killed_vecs.is_empty() {
-            let src = killed_vecs.pop().unwrap();
+        while let Some(src) = killed_vecs.pop() {
             if src.comps() >= vec_dst.comps {
                 vec_dst.killed = Some(src);
                 break;

src/nouveau/compiler/nak/lower_par_copies.rs

@@ -143,8 +143,7 @@ fn lower_par_copy(pc: OpParCopy, sm: u8) -> MappedInstrs {
             ready.push(i);
         }
     }
-    while !ready.is_empty() {
-        let dst_idx = ready.pop().unwrap();
+    while let Some(dst_idx) = ready.pop() {
         if let Some(src_idx) = graph.src(dst_idx) {
             let dst = *vals[dst_idx].as_reg().unwrap();
             let src = vals[src_idx];