nir: Add lowering support for packing opcodes.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>

src/compiler/nir/nir.h

@@ -1469,6 +1469,10 @@ typedef struct nir_shader_compiler_options {
    bool lower_ffract;
 
    bool lower_pack_half_2x16;
+   bool lower_pack_unorm_2x16;
+   bool lower_pack_snorm_2x16;
+   bool lower_pack_unorm_4x8;
+   bool lower_pack_snorm_4x8;
    bool lower_unpack_half_2x16;
 
    bool lower_extract_byte;