nir/lower_int64: Fix compilation with gcc-13 and tsan enabled

../src/compiler/nir/nir_lower_int64.c: In function ‘lower_int64_intrinsic’:
../src/compiler/nir/nir_lower_int64.c:1347:1: error: control reaches end of non-void function [-Werror=return-type]
1347 | }

Fixes: bf7a114246
   nir/lower_int64: Add lowering for some 64-bit subgroup ops

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27345>

src/compiler/nir/nir_lower_int64.c

@@ -1344,6 +1344,7 @@ lower_int64_intrinsic(nir_builder *b, nir_intrinsic_instr *intrin)
    default:
       unreachable("Unsupported intrinsic");
    }
+   return NULL;
 }
 
 static bool