LoongArch: Simplify vec_widen_<su>{add/sub/mult}_{hi/lo}_m describe.

LoongArch: Simplify vec_widen_<su>{add/sub/mult}_{hi/lo}_m describe.

gcc/ChangeLog:

	* config/loongarch/lasx.md (vec_widen_<su>add_hi_<mode>):
	Move.
	(vec_widen_<su>add_lo_<mode>): Move.
	(vec_widen_<su>sub_hi_<mode>): Move.
	(vec_widen_<su>sub_lo_<mode>): Move.
	(vec_widen_<su>mult_hi_<mode>): Move.
	(vec_widen_<su>mult_lo_<mode>): Move.
	(hi_lo): New define_int_attr.
	(vec_widen_<su><optab>_<hi_lo>_<mode>): New define_expand.
	(vec_widen_<su>mult_<hi_lo>_<mode>): Likewise.
	* config/loongarch/loongarch-protos.h
	(loongarch_expand_vec_widen_hilo): Modify the function
	parameter list.
	* config/loongarch/loongarch.cc
	(loongarch_expand_vec_widen_hilo): Optimized.