LoongArch: Add prefetch instructions.

LoongArch: Add prefetch instructions.

Enable sw prefetching at -O3 and higher.

Co-Authored-By: xujiahao <xujiahao@loongson.cn>
gcc/ChangeLog:

	* config/loongarch/constraints.md (ZD): New constraint.
	* config/loongarch/loongarch-def.c: Initial number of parallel prefetch.
	* config/loongarch/loongarch-tune.h (struct loongarch_cache):
	Define number of parallel prefetch.
	* config/loongarch/loongarch.cc (loongarch_option_override_internal):
	Set up parameters to be used in prefetching algorithm.
	* config/loongarch/loongarch.md (prefetch): New template.