glsl: Add support for lowering 4x8 pack/unpack operations

Lower them to arithmetic and bit manipulation expressions.

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>

src/glsl/ir_optimization.h

@@ -54,6 +54,12 @@ enum lower_packing_builtins_op {
 
    LOWER_PACK_HALF_2x16_TO_SPLIT        = 0x0040,
    LOWER_UNPACK_HALF_2x16_TO_SPLIT      = 0x0080,
+
+   LOWER_PACK_SNORM_4x8                 = 0x0100,
+   LOWER_UNPACK_SNORM_4x8               = 0x0200,
+
+   LOWER_PACK_UNORM_4x8                 = 0x0400,
+   LOWER_UNPACK_UNORM_4x8               = 0x0800,
 };
 
 bool do_common_optimization(exec_list *ir, bool linked,