nir/lower_tex: Add an assert() in nir_lower_txs_lod()

nir/lower_tex: Add an assert() in nir_lower_txs_lod()

We don't expect the output of a TXS instruction to be wider than a
vec3. Add an assert() to make sure this never happens.

Suggested-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>