[RISCV] Use a constraint for bset<mode>_mask and bset<mode>_1_mask

[RISCV] Use a constraint for bset<mode>_mask and bset<mode>_1_mask

A constraint here just makes it easier to understand what the
operands are.

OK? Built and tested on riscv32-linux-gnu and riscv64-linux-gnu with
--with-arch=rvNimafdc_zba_zbb_zbc_zbs (where N is 32 and 64).

Thanks,
Andrew Pinski

gcc/ChangeLog:

	* config/riscv/constraints.md (DsS): New constraint.
	(DsD): New constraint.
	* config/riscv/iterators.md (shiftm1c): New iterator.
	* config/riscv/bitmanip.md (*bset<mode>_mask):
	Use shiftm1c.
	(*bset<mode>_1_mask): Likewise.