AArch64: Cleanup memset expansion

Cleanup memset implementation.  Similar to memcpy/memmove, use an offset and
bytes throughout.  Simplify the complex calculations when optimizing for size
by using a fixed limit.

gcc/ChangeLog:
	* config/aarch64/aarch64.cc (MAX_SET_SIZE): New define.
	(aarch64_progress_pointer): Remove function.
	(aarch64_set_one_block_and_progress_pointer): Simplify and clean up.
	(aarch64_expand_setmem): Clean up implementation, use byte offsets,
	simplify size calculation.