LoongArch: Improve cpymemsi expansion [PR109465]

We'd been generating really bad block move sequences which is recently
complained by kernel developers who tried __builtin_memcpy.  To improve
it:

1. Take the advantage of -mno-strict-align.  When it is set, set mode
   size to UNITS_PER_WORD regardless of the alignment.
2. Half the mode size when (block size) % (mode size) != 0, instead of
   falling back to ld.bu/st.b at once.
3. Limit the length of block move sequence considering the number of
   instructions, not the size of block.  When -mstrict-align is set and
   the block is not aligned, the old size limit for straight-line
   implementation (64 bytes) was definitely too large (we don't have 64
   registers anyway).

Change since v1: add a comment about the calculation of num_reg.

gcc/ChangeLog:

	PR target/109465
	* config/loongarch/loongarch-protos.h
	(loongarch_expand_block_move): Add a parameter as alignment RTX.
	* config/loongarch/loongarch.h:
	(LARCH_MAX_MOVE_BYTES_PER_LOOP_ITER): Remove.
	(LARCH_MAX_MOVE_BYTES_STRAIGHT): Remove.
	(LARCH_MAX_MOVE_OPS_PER_LOOP_ITER): Define.
	(LARCH_MAX_MOVE_OPS_STRAIGHT): Define.
	(MOVE_RATIO): Use LARCH_MAX_MOVE_OPS_PER_LOOP_ITER instead of
	LARCH_MAX_MOVE_BYTES_PER_LOOP_ITER.
	* config/loongarch/loongarch.cc (loongarch_expand_block_move):
	Take the alignment from the parameter, but set it to
	UNITS_PER_WORD if !TARGET_STRICT_ALIGN.  Limit the length of
	straight-line implementation with LARCH_MAX_MOVE_OPS_STRAIGHT
	instead of LARCH_MAX_MOVE_BYTES_STRAIGHT.
	(loongarch_block_move_straight): When there are left-over bytes,
	half the mode size instead of falling back to byte mode at once.
	(loongarch_block_move_loop): Limit the length of loop body with
	LARCH_MAX_MOVE_OPS_PER_LOOP_ITER instead of
	LARCH_MAX_MOVE_BYTES_PER_LOOP_ITER.
	* config/loongarch/loongarch.md (cpymemsi): Pass the alignment
	to loongarch_expand_block_move.

gcc/testsuite/ChangeLog:

	PR target/109465
	* gcc.target/loongarch/pr109465-1.c: New test.
	* gcc.target/loongarch/pr109465-2.c: New test.
	* gcc.target/loongarch/pr109465-3.c: New test.