nir_to_tgsi: Extract const components of atomic counter offsets into Index.

nir_to_tgsi: Extract const components of atomic counter offsets into Index.

virglrenderer maps atomic accesses to atomic counter declarations using
the .Index field.  We were previously emitting a .Index of 0 for array
accesses, so virglrenderer would emit
atomicIncrement(first_counter[counter_offset+array_index]).  This would
mostly work because hardware doesn't care about the bounds of counter
declarations, but if the first counter was a non-array, then the [] GLSL
emit gets dropped (can't array access a scalar!) and you'd access the
non-array first_counter instead.

Acked-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15824>