nak: Compute liveness in reverse block order

If we walk the blocks forwards, we end up having a minimum O(n^2)
algorithm because everything has to propagate bottom to top.  Looping
over the blocks bacwards ensures that all the liveness information is
propgated in the first pass in the absence of back edges.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24998>

src/nouveau/compiler/nak_liveness.rs

@@ -140,7 +140,7 @@ impl Liveness {
         let mut to_do = true;
         while to_do {
             to_do = false;
-            for b in &func.blocks {
+            for b in func.blocks.iter().rev() {
                 let bl = l.blocks.get_mut(&b.id).unwrap();
 
                 /* Compute live-out */