aarch64: Remove redundant costing code

Previous patches made some of the complex parts of the issue rate
code redundant.

gcc/
	* config/aarch64/aarch64.c (aarch64_vector_op::n_advsimd_ops): Delete.
	(aarch64_vector_op::m_seen_loads): Likewise.
	(aarch64_vector_costs::aarch64_vector_costs): Don't push to
	m_advsimd_ops.
	(aarch64_vector_op::count_ops): Remove vectype and factor parameters.
	Remove code that tries to predict different vec_flags from the
	current loop's.
	(aarch64_vector_costs::add_stmt_cost): Update accordingly.
	Remove m_advsimd_ops handling.