nir: add array lowering function that assumes there are no indirects

nir: add array lowering function that assumes there are no indirects

The gallium glsl->nir pass currently lowers away all indirects on both inputs
and outputs. This fuction allows us to lower vs inputs and fs outputs and also
lower things one stage at a time as we don't need to worry about indirects
on the other side of the shaders interface.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>