nv50/ir/nir: ignore sampler for TXF/TXQ ops.

Recently, a regression was reported where videos in Firefox had shifted/
glitched colors on certain Kepler hardware. This was bisected to
bf02bffe15, however, the issue already
existed but didn't hit users until TGSI was switched to NIR as default.

The issue was traced to a YUV-to-RGB fragment shader used by Firefox,
which uses three samplers for the Y/U/V components. The Y component was
handled correctly, but the U/V components were bogus, causing the issue.

After analysis, it appears the TXF/TXQ ops. should only handle the texture
(r) but not the sampler (s), see 63b850403c
and 346ce0b988.
Similarly, handleTXQ/handleTXF on nv50_ir_from_tgsi always sets s=0.
Only Kepler was affected because other hardware ignores s at codegen.

Always set s=0 on NIR for TXF/TXQ, to keep TGSI behavior and fix the
regression.

Thanks: Karol Herbst and M Henning for help diagnosing the issue.
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/7416
Cc: mesa-stable
Suggested-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: M Henning <drawoc@darkrefraction.com>
Signed-off-by: Joan Bruguera <joanbrugueram@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19453>

src/nouveau/codegen/nv50_ir_from_nir.cpp

@@ -3133,6 +3133,8 @@ Converter::visit(nir_tex_instr *insn)
 
       r = bindless ? 0xff : insn->texture_index;
       s = bindless ? 0x1f : insn->sampler_index;
+      if (op == OP_TXF || op == OP_TXQ)
+         s = 0;
 
       defs.resize(newDefs.size());
       for (uint8_t d = 0u; d < newDefs.size(); ++d) {