nak: set max_gpr to multiple of 8s

Optimizations cutting down on GPRs often lead to the akward situations
where RA being more restricted and having to insert more mov instructions
pumping up the instruction counts.

In order to give developers more reliable stats we just set the max_gprs
to the next multiple of 8 including taking hw reserved registers into
account.

This does not impact occupancy in any way despite the increase in gprs.

Totals:
CodeSize: 920980864 -> 914748784 (-0.68%); split: -0.69%, +0.02%
Number of GPRs: 3544248 -> 3879749 (+9.47%)
Static cycle count: 217345431 -> 216414194 (-0.43%); split: -0.50%, +0.07%

Totals from 78493 (89.58% of 87622) affected shaders:
CodeSize: 795883088 -> 789651008 (-0.78%); split: -0.80%, +0.02%
Number of GPRs: 3108571 -> 3444072 (+10.79%)
Static cycle count: 187450578 -> 186519341 (-0.50%); split: -0.58%, +0.08%

Reviewed-by: Mel Henning <mhenning@darkrefraction.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36514>

src/nouveau/compiler/nak/assign_regs.rs

@@ -1472,11 +1472,12 @@ impl Shader<'_> {
             max_gprs += 2;
         }
 
+        let hw_reserved_gprs = self.sm.hw_reserved_gprs();
         if let ShaderStageInfo::Compute(cs_info) = &self.info.stage {
             max_gprs = min(
                 max_gprs,
                 gpr_limit_from_local_size(&cs_info.local_size)
-                    - self.sm.hw_reserved_gprs(),
+                    - hw_reserved_gprs,
             );
         }
 
@@ -1492,6 +1493,17 @@ impl Shader<'_> {
 
             // Re-calculate liveness one last time
             live = SimpleLiveness::for_function(f);
+        } else {
+            // GPRs are allocated in multiple of 8. That means we can give RA a
+            // bit more freedom by making gprs up until the next multiple
+            // available.
+            let next_multiple_gprs = (total_gprs + hw_reserved_gprs)
+                .next_multiple_of(8)
+                - hw_reserved_gprs;
+            let free_gprs = next_multiple_gprs.min(max_gprs) - total_gprs;
+
+            total_gprs += free_gprs;
+            gpr_limit += free_gprs;
         }
 
         self.info.num_gprs = total_gprs.try_into().unwrap();