[RISCV] Add constraints for not_single_bit_mask_operand/single_bit_mask_operand

[RISCV] Add constraints for not_single_bit_mask_operand/single_bit_mask_operand

Like a previous patch, just add constraints for predicates
not_single_bit_mask_operand and single_bit_mask_operand.

OK? Built and tested for riscv32-linux-gnu and riscv64-linux-gnu.

Thanks,
Andrew Pinski

gcc/ChangeLog:

	* config/riscv/constraints.md (DbS): New constraint.
	(DnS): New constraint.
	* config/riscv/bitmanip.md (*bset<mode>_1_mask): Use new constraint.
	(*bclr<mode>): Likewise.
	(*binvi<mode>): Likewise.