nir: Rewrite and merge 16bit tex folding pass with 16bit image folding pass.

Allow folding constants/undef sources by sharing more code with the image_store 16bit folding pass.

Allow more than one set of sources because RADV wants two, one for G16 (ddx/ddy) and one for A16 (all other sources).

Allow folding cube sampling destination conversions on radeonsi/radv because I think the limitation only applies to sources.