ac/nir: Handle const array offsets in get_deref_offset()

ac/nir: Handle const array offsets in get_deref_offset()

Some users of this function (e.g. GS inputs) currently only work with
constant offsets. We got lucky since all the tests used an array index
of 0, so the non-constant part was always 0. But we still need to handle
this.

This doesn't fix any CTS test, but was noticed while debugging one.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>