nir: Add nir_lower_tex options to lower sampler return formats.

nir: Add nir_lower_tex options to lower sampler return formats.

I've been doing this in the nir-to-vir and nir-to-qir backends of v3d and
vc4, but nir could potentially do some useful stuff for us (like avoiding
unpack/repacks) if we give it the information.

v2: Skip lowering for txs/query_levels
v3: Fix a crash on old-style shadow
v4: Rename to tex_packing, use nir_format_unpack_sint/uint helpers, pack
    the enum.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>