nak: Allow SSA value 0 again

We no longer need the index to be non-zero because we add 17.

Reviewed-by: Mel Henning <mhenning@darkrefraction.com>
Acked-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/41462>

src/nouveau/compiler/nak/ssa_value.rs

@@ -306,8 +306,9 @@ impl SSAValueAllocator {
 
     /// Allocates an SSA value.
     pub fn alloc(&mut self, file: RegFile) -> SSAValue {
+        let idx = self.count;
         self.count += 1;
-        SSAValue::new(file, self.count)
+        SSAValue::new(file, idx)
     }
 
     /// Allocates multiple SSA values and returns them as an SSA reference.