RISC-V: Add fixed-point support

RISC-V: Add fixed-point support

gcc/ChangeLog:

	* config/riscv/riscv-vector-builtins-bases.cc (class sat_op): New class.
	(class vnclip): Ditto.
	(BASE): Ditto.
	* config/riscv/riscv-vector-builtins-bases.h: Ditto.
	* config/riscv/riscv-vector-builtins-functions.def (vaadd): Ditto.
	(vasub): Ditto.
	(vaaddu): Ditto.
	(vasubu): Ditto.
	(vsmul): Ditto.
	(vssra): Ditto.
	(vssrl): Ditto.
	(vnclipu): Ditto.
	(vnclip): Ditto.
	* config/riscv/vector-iterators.md (su): Add instruction.
	(aadd): Ditto.
	(vaalu): Ditto.
	* config/riscv/vector.md (@pred_<sat_op><mode>): New pattern.
	(@pred_<sat_op><mode>_scalar): Ditto.
	(*pred_<sat_op><mode>_scalar): Ditto.
	(*pred_<sat_op><mode>_extended_scalar): Ditto.
	(@pred_narrow_clip<v_su><mode>): Ditto.
	(@pred_narrow_clip<v_su><mode>_scalar): Ditto.