glsl: Extend ir_expression_operation for ARB_shading_language_packing

glsl: Extend ir_expression_operation for ARB_shading_language_packing

For each function {pack,unpack}{Snorm,Unorm}4x8, add a corresponding
opcode to enum ir_expression_operation. Validate the new opcodes in
ir_validate.cpp.

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