R600/nir: lower cube texture ops and merge lod, bias, and comparison into coordinate
The MR updates the texture lookup instrution to be able to handle cube textures lowered to texture arrays and to merge lod, bias, and shadow comparison values into the coordinate source component. With that in place lowering passes are added that lower cube texture lookup to texture array lookups and that merge lod/bias/comparison into the the coordinate source value.
By lowering this on the NIR level some optimizations are made possible, namely some useless copies are avoided.