aarch64: Implement determine_suggested_unroll_factor

aarch64: Implement determine_suggested_unroll_factor

This patch implements the costing function determine_suggested_unroll_factor
for aarch64.
It determines the unrolling factor by dividing the number of X operations we
can do per cycle by the number of X operations, taking this information from
the vec_ops analysis during vector costing and the available issue_info
information.
We multiply the dividend by a potential reduction_latency, to improve our
pipeline utilization if we are stalled waiting on a particular reduction
operation.

gcc/ChangeLog:

	* config/aarch64/aarch64.cc (aarch64_vector_costs): Define
	determine_suggested_unroll_factor and m_has_avg.
	(determine_suggested_unroll_factor): New function.
	(aarch64_vector_costs::add_stmt_cost): Check for a qualifying pattern
	to set m_nosve_pattern.
	(aarch64_vector_costs::finish_costs): Use
	determine_suggested_unroll_factor.
	* config/aarch64/aarch64.opt (aarch64-vect-unroll-limit): New.
	* doc/invoke.texi: (aarch64-vect-unroll-limit): Document new option.