intel/ir: Represent physical edge of unconditional CONTINUE instruction.

intel/ir: Represent physical edge of unconditional CONTINUE instruction.

This edge doesn't exist in the original scalar program, but it
represents a potential control flow path the EU will take in cases
where control flow isn't uniform across channels of the same SIMD
thread.

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