aarch64: Add vf_factor to aarch64_vec_op_count

aarch64: Add vf_factor to aarch64_vec_op_count

-mtune=neoverse-512tvb sets the likely SVE vector length to 128 bits,
but it also takes into account Neoverse V1, which is a 256-bit target.
This patch adds this VF (VL) factor to aarch64_vec_op_count.

gcc/
	* config/aarch64/aarch64.c (aarch64_vec_op_count::m_vf_factor):
	New member variable.
	(aarch64_vec_op_count::aarch64_vec_op_count): Add a parameter for it.
	(aarch64_vec_op_count::vf_factor): New function.
	(aarch64_vector_costs::aarch64_vector_costs): When costing for
	neoverse-512tvb, pass a vf_factor of 2 for the Neoverse V1 version
	of an SVE loop.
	(aarch64_vector_costs::adjust_body_cost): Read the vf factor
	instead of hard-coding 2.