RISC-V: Add saturating Addition && Subtraction C/C++ Support

RISC-V: Add saturating Addition && Subtraction C/C++ Support

gcc/ChangeLog:

	* config/riscv/iterators.md: Add saturating Addition && Subtraction.
	* config/riscv/riscv-v.cc (has_vi_variant_p): Ditto.
	* config/riscv/riscv-vector-builtins-bases.cc (BASE): Ditto.
	* config/riscv/riscv-vector-builtins-bases.h: Ditto.
	* config/riscv/riscv-vector-builtins-functions.def (vsadd): New def.
	(vssub): Ditto.
	(vsaddu): Ditto.
	(vssubu): Ditto.
	* config/riscv/vector-iterators.md (sll.vi): Adjust for Saturating
	support.
	(sll.vv): Ditto.
	(%3,%v4): Ditto.
	(%3,%4): Ditto.
	* config/riscv/vector.md (@pred_<optab><mode>): New pattern.
	(@pred_<optab><mode>_scalar): New pattern.
	(*pred_<optab><mode>_scalar): New pattern.
	(*pred_<optab><mode>_extended_scalar): New pattern.