nir: add two new texture ops for multisample fragment color/mask fetches

nir: add two new texture ops for multisample fragment color/mask fetches

This introduces:
   - nir_texop_fragment_mask_fetch (fetch a fragment mask from a
     compressed multisampled color surface)
   - nir_texop_fragment_fetch (fetch a color fragment for a
     particular sample at corresponding fragment mask index).

These two texture operations are necessary for implementing
SPV_AMD_shader_fragment_mask.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3304>