nir/vec_to_movs: Don't generate MOVs for undef channels.

nir/vec_to_movs: Don't generate MOVs for undef channels.

This appeared in softpipe's image operations, since NIR always uses
4-component values for the coords, while the GLSL IR only has 2 components
for a 2D image (for example).
arb_shader_image_load_store-shader-mem-barrier (which times out in CI and
spends its time inside of tgsi_exec) was spending 4/51 of its instructions
on moving these undefs around.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9345>