nir/print: Don't print sampler_index for txf

nir/print: Don't print sampler_index for txf

NIR's docs for sampler_index say

    The following operations do not require a sampler and, as such, this
    field should be ignored:
       - nir_texop_txf
       - nir_texop_txf_ms
       - nir_texop_txs
       - nir_texop_query_levels
       - nir_texop_texture_samples
       - nir_texop_samples_identical

Contrary to this documentation, we were still printing the sampler_index anyway,
even though the value is formally undefined. This was helpful for
PIPE_CAP_TEXTURE_BUFFER_SAMPLER drivers that (despite the NIR docs) respected the
sampler_index anyway. There are no longer any such drivers, so we should stop
printing sampler_index for txf to avoid confusion (and noise).

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/22223>