GCN: Remove 'FIRST_{SGPR,VGPR,AVGPR}_REG', 'LAST_{SGPR,VGPR,AVGPR}_REG' from machine description

They're not used there, and we avoid potentially out-of-sync definitions.

	gcc/
	* config/gcn/gcn.md (FIRST_SGPR_REG, LAST_SGPR_REG)
	(FIRST_VGPR_REG, LAST_VGPR_REG, FIRST_AVGPR_REG, LAST_AVGPR_REG):
	Don't 'define_constants'.