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

This appeared in softpipe's image stores, 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 in tgsi_exec parsing) was spending 4/51 of its instructions on moving these undefs around.