intel/compiler: Add a INVALID_{,HW_}REG_TYPE macros

intel/compiler: Add a INVALID_{,HW_}REG_TYPE macros

Since the enum brw_reg_type is packed, comparisons with -1 don't work
directly, necessitating the cast. Add a macro to avoid this confusion.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2635>