intel/fs: Define is_payload() method of the IR instruction class.

intel/fs: Define is_payload() method of the IR instruction class.

This is required because SEND message payload sources are fetched
asynchronously by the hardware, which can lead to WaR data corruption
on Gen12+ platforms if not handled specially by the compiler to
guarantee proper synchronization.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>