ac/nir: Fix conflict resolution typo in handle_vs_input_decl

Fixes ad764e36:
"ac/nir: Use instance_rate_inputs per attribute, not per variable".

CC: <mesa-stable@lists.freedesktop.org>
CC: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>