nir/lower_io: Set interpolated_input dest_type

nir/lower_io: Set interpolated_input dest_type

...even for non-pixel interpolation, for consistency. Otherwise backends get
funny intrinsics with interpolateAt:

   vec4 32 ssa_4 = intrinsic load_interpolated_input (ssa_3, ssa_2) (base=1, component=0, dest_type=invalid /*0*/, io location=33 slots=1 /*161*/)

We know it'll be a float, but backends shouldn't need to special case this. (Or
maybe interpolated_input shouldn't have a dest_type index. I'd be ok with that
resolution too. But having one and not setting it consistently is wrong.)

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19085>