nak/hw_tests: Respect src_mod and src_swizzle in op auto-tests

Instead of replacing the whole source, just replace the src_ref so we
maintain the modifier.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30402>

src/nouveau/compiler/nak/hw_tests.rs

@@ -329,7 +329,7 @@ pub fn test_foldable_op_with(
                 let data = b.ld_test_data(comps * 4, MemType::B32);
                 comps += 1;
 
-                *src = data.into();
+                src.src_ref = data.into();
                 fold_src.push(FoldData::U32(0));
             }
             SrcType::F64 => {
@@ -346,7 +346,7 @@ pub fn test_foldable_op_with(
                     bit.into(),
                     0.into(),
                 );
-                *src = pred.into();
+                src.src_ref = pred.into();
                 fold_src.push(FoldData::Pred(false));
             }
             typ => panic!("Can't auto-generate {typ:?} data"),