nak: Use undef for unset FSOut components

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27742>

src/nouveau/compiler/nak/builder.rs

@@ -652,6 +652,12 @@ pub trait SSABuilder: Builder {
         }
     }
 
+    fn undef(&mut self) -> SSARef {
+        let dst = self.alloc_ssa(RegFile::GPR, 1);
+        self.push_op(OpUndef { dst: dst.into() });
+        dst
+    }
+
     fn copy(&mut self, src: Src) -> SSARef {
         let dst = if src.is_predicate() {
             self.alloc_ssa(RegFile::Pred, 1)

src/nouveau/compiler/nak/from_nir.rs

@@ -2249,7 +2249,7 @@ impl<'a> ShaderFromNir<'a> {
                         for c in 0..4 {
                             let reg = self.fs_out_regs[i * 4 + c];
                             if reg.is_none() {
-                                srcs.push(0.into());
+                                srcs.push(b.undef().into());
                             } else {
                                 srcs.push(reg.into());
                             }
@@ -2262,7 +2262,7 @@ impl<'a> ShaderFromNir<'a> {
                     if info.writes_sample_mask {
                         srcs.push(self.fs_out_regs[mask_idx].into());
                     } else {
-                        srcs.push(0.into());
+                        srcs.push(b.undef().into());
                     }
                     if info.writes_depth {
                         // Saturate depth writes.