LoongArch:Implement vec_widen standard names.

Add support for vec_widen lo/hi patterns.  These do not directly
match on Loongarch lasx instructions but can be emulated with
even/odd + vector merge.

gcc/ChangeLog:

	* config/loongarch/lasx.md
	(vec_widen_<su>mult_even_v8si): New patterns.
	(vec_widen_<su>add_hi_<mode>): Ditto.
	(vec_widen_<su>add_lo_<mode>): Ditto.
	(vec_widen_<su>sub_hi_<mode>): Ditto.
	(vec_widen_<su>sub_lo_<mode>): Ditto.
	(vec_widen_<su>mult_hi_<mode>): Ditto.
	(vec_widen_<su>mult_lo_<mode>): Ditto.
	* config/loongarch/loongarch.md (u_bool): New iterator.
	* config/loongarch/loongarch-protos.h
	(loongarch_expand_vec_widen_hilo): New prototype.
	* config/loongarch/loongarch.cc
	(loongarch_expand_vec_interleave): New function.
	(loongarch_expand_vec_widen_hilo): New function.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/vect-widen-add.c: New test.
	* gcc.target/loongarch/vect-widen-mul.c: New test.
	* gcc.target/loongarch/vect-widen-sub.c: New test.