st/glsl: fix broken vertex attrib mapping

st/glsl: fix broken vertex attrib mapping

Here we move the nir_get_single_slot_attribs_mask() call that sets the
inputs_read mask after NIR optimisations have finished and after
st_nir_assign_vs_in_locations() has been called.

Besides fixing a bug where the mappings would be missaligned if
further NIR optimisations resulted in less inputs being read, it
also allows us to drop an additional nir gather info call.

Fixes: 0909a57b631f ("radeonsi/nir: Set vs_inputs_dual_locations and let NIR do the remap")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/6240

Reviewed-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17326>