nir/lower_tex: Add AYUV lowering support

Byte ordering is :

0: V
1: U
2: Y
3: A

v2: Split refactoring of alpha channel (Lionel)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com> (v1)
Acked-by: Eric Engestrom <eric.engestrom@intel.com> (v2)

src/compiler/nir/nir.h

@@ -2905,6 +2905,7 @@ typedef struct nir_lower_tex_options {
    unsigned lower_y_u_v_external;
    unsigned lower_yx_xuxv_external;
    unsigned lower_xy_uxvx_external;
+   unsigned lower_ayuv_external;
 
    /**
     * To emulate certain texture wrap modes, this can be used