nir: Don't handle nir_op_mov in get_undef_mask in opt_undef

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: 5f26c21e6246 ("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>