LoongArch: Implement vector reduction from 256-bit to 128-bit

LoongArch: Implement vector reduction from 256-bit to 128-bit

gcc/ChangeLog:

	* config/loongarch/lasx.md (vec_extract<mode><lasxhalf>): New define_expand.
	(vec_extract_lo_<mode>): New define_insn_and_split.
	(vec_extract_hi_<mode>): New define_insn.
	* config/loongarch/loongarch-protos.h (loongarch_check_vect_par_cnst_half)
	New function prototype.
	* config/loongarch/loongarch.cc (loongarch_split_reduction):
	Implement TARGET_VECTORIZE_SPLIT_REDUCTION.
	(loongarch_check_vect_par_cnst_half): New function.
	* config/loongarch/predicates.md
	(vect_par_cnst_low_half): New predicate.
	(vect_par_cnst_high_half): New predicate.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/lasx-reduc-1.c: New test.