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>
(cherry picked from commit 80a1b91601)

.pick_status.json

@@ -644,7 +644,7 @@
         "description": "nir/lower_int64: Fix compilation with gcc-13 and tsan enabled",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "bf7a11424650af2ac9e8f546741fb6ab2ed3b8d7",
         "notes": null

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