nak: Run simple liveness data-flow bottom-up

Rookie mistake... The liveness algorithm propagates information from
later blocks to earlier blocks so if you run bottom-up it's exactly two
passes when there aren't loops.  If you run top-down, it's quadratic in
the number of blocks.

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

src/nouveau/compiler/nak_liveness.rs

@@ -314,7 +314,7 @@ impl SimpleLiveness {
         let mut to_do = true;
         while to_do {
             to_do = false;
-            for (b_idx, bl) in l.blocks.iter_mut().enumerate() {
+            for (b_idx, bl) in l.blocks.iter_mut().enumerate().rev() {
                 /* Compute live-out */
                 for sb_idx in func.blocks.succ_indices(b_idx) {
                     to_do |= bl.live_out.union_with(&live_in[*sb_idx]);