nak: Forbid reordering labeled OpNop

Totals:
Static cycle count: 1104322907 -> 1108862573 (+0.41%)

Totals from 111376 (56.68% of 196502) affected shaders:
Static cycle count: 948085895 -> 952625561 (+0.48%)

Fixes: 79d0f8263d ("nak: Add a simple postpass instruction scheduler")
Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35141>

src/nouveau/compiler/nak/opt_instr_sched_common.rs

@@ -211,7 +211,14 @@ pub fn side_effect_type(op: &Op) -> SideEffect {
         | Op::ViLd(_)
         | Op::Kill(_)
         | Op::S2R(_) => SideEffect::Barrier,
-        Op::PixLd(_) | Op::Nop(_) | Op::Vote(_) => SideEffect::None,
+        Op::PixLd(_) | Op::Vote(_) => SideEffect::None,
+        Op::Nop(OpNop { label, .. }) => {
+            if label.is_none() {
+                SideEffect::None
+            } else {
+                SideEffect::Barrier
+            }
+        }
 
         // Virtual ops
         Op::Annotate(_)