LoongArch: Use LSX and LASX for block move

LoongArch: Use LSX and LASX for block move

gcc/ChangeLog:

	* config/loongarch/loongarch.h (LARCH_MAX_MOVE_PER_INSN):
	Define to the maximum amount of bytes able to be loaded or
	stored with one machine instruction.
	* config/loongarch/loongarch.cc (loongarch_mode_for_move_size):
	New static function.
	(loongarch_block_move_straight): Call
	loongarch_mode_for_move_size for machine_mode to be moved.
	(loongarch_expand_block_move): Use LARCH_MAX_MOVE_PER_INSN
	instead of UNITS_PER_WORD.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/memcpy-vec-1.c: New test.
	* gcc.target/loongarch/memcpy-vec-2.c: New test.
	* gcc.target/loongarch/memcpy-vec-3.c: New test.