ac/nir_to_llvm: fix interpolateAt* for arrays

This builds on the recent interpolate fix by Rhys ee8488ea.

This fixes the arb_gpu_shader5 interpolateAt* tests that contain
arrays.

Fixes: ee8488ea ("ac/nir,radv,radeonsi/nir: use correct indices for interpolation intrinsics")

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit 9e669ed2)