intel/ir: Allow representing additional flag subregisters in the IR.

intel/ir: Allow representing additional flag subregisters in the IR.

This allows representing conditional mods and predicates on f1.0-f1.1
at the IR level by adding an extra bit to the flag_subreg
backend_instruction field.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>