nir/lower_input_attachments: remove bogus assert in try_lower_input_texop()

nir/lower_input_attachments: remove bogus assert in try_lower_input_texop()

It can be a sampler too.

Fixes: 84b08971fbd ("nir/lower_input_attachments: lower nir_texop_fragment_{mask}_fetch")
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2558
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4043>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4043>