nir: Don't handle nir_op_mov in get_undef_mask in opt_undef

It's unnecessary because earlier parts of the pass will ensure that a
mov of undef is turned into an undef.  It's also wrong because
nir_op_mov has different semantics from nir_op_vecN when it comes to how
sources map to destination components.

Fixes: 5f26c21e62 ("nir: Expand opt_undef to handle undef channels in a store intrinsic")
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24704>
(cherry picked from commit 408929289a)

.pick_status.json

@@ -6994,7 +6994,7 @@
         "description": "nir: Don't handle nir_op_mov in get_undef_mask in opt_undef",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "5f26c21e62466783cd031c45a39544c992cee202",
         "notes": null

src/compiler/nir/nir_opt_undef.c

@@ -110,7 +110,8 @@ nir_get_undef_mask(nir_ssa_def *def)
    nir_alu_instr *alu = nir_instr_as_alu(instr);
    unsigned undef = 0;
 
-   if (nir_op_is_vec(alu->op)) {
+   /* nir_op_mov of undef is handled by opt_undef_vecN() */
+   if (nir_op_is_vec(alu->op) && alu->op != nir_op_mov) {
       for (int i = 0; i < nir_op_infos[alu->op].num_inputs; i++) {
          if (alu->src[i].src.is_ssa &&
              alu->src[i].src.ssa->parent_instr->type ==