[RISCV] Use constraints/predicates instead of checking const_int directly for shNadd...

[RISCV] Use constraints/predicates instead of checking const_int directly for shNadd patterns

This simplifies the code by adding a predicate and a constraint for 1/2/3.
The aarch64 backend has a similar predicate called aarch64_shift_imm_<mode>
which they use there.

OK? Built and tested on riscv32-linux-gnu and riscv64-linux-gnu with no regressions.

Thanks,
Andrew Pinski

gcc/ChangeLog:

	* config/riscv/constraints.md (Ds3): New constraint.
	* config/riscv/predicates.md (imm123_operand): New predicate.
	* config/riscv/bitmanip.md (*shNadd): Use Ds3 and imm123_operand.
	(*shNadduw): Likewise.