ntt: remove dead input variables before lowering FS IO

ntt: remove dead input variables before lowering FS IO

Because for fragment shaders we still use the variables, and
lower_io_to_vector may leave dead variables that duplicate inputs
that are now vectorized, we have to call this pass, because otherwise
we will may hit the assertion

src/gallium/auxiliary/tgsi/tgsi_ureg.c:318:
  ureg_DECL_fs_input_centroid_layout:
   Assertion `(ureg->input[i].usage_mask & usage_mask) == 0'

This is relevant for
  spec@arb_enhanced_layouts@execution@component-layout@*
on r600/ntt

Fixes: a4840e15ab77b44a72cabd7d503172e8357477eb
  r600: Use nir-to-tgsi instead of TGSI when the NIR debug opt is disabled

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16130>