aarch64: Try to detect when Advanced SIMD code would be completely unrolled

aarch64: Try to detect when Advanced SIMD code would be completely unrolled

GCC usually costs the SVE and Advanced SIMD versions of a loop
and picks the one with the lowest cost.  By default it will choose
SVE over Advanced SIMD in the event of tie.

This is normally the correct behaviour, not least because SVE can
handle every scalar iteration count whereas Advanced SIMD can only
handle full vectors.  However, there is one important exception
that GCC failed to consider: we can completely unroll Advanced SIMD
code at compile time, but we can't do the same for SVE.

This patch therefore adds an opt-in heuristic to guess whether
the Advanced SIMD version of a loop is likely to be unrolled.
This will only be suitable for some CPUs, so it is not enabled
by default and is controlled separately from use_new_vector_costs.

Like with previous patches, this one only becomes active if a
CPU selects both of the new tuning parameters.  It should therefore
have a very low impact on other CPUs.

gcc/
	* config/aarch64/aarch64-tuning-flags.def (matched_vector_throughput):
	New tuning parameter.
	* config/aarch64/aarch64.c (neoversev1_tunings): Use it.
	(aarch64_estimated_sve_vq): New function.
	(aarch64_vector_costs::analyzed_vinfo): New member variable.
	(aarch64_vector_costs::is_loop): Likewise.
	(aarch64_vector_costs::unrolled_advsimd_niters): Likewise.
	(aarch64_vector_costs::unrolled_advsimd_stmts): Likewise.
	(aarch64_record_potential_advsimd_unrolling): New function.
	(aarch64_analyze_loop_vinfo, aarch64_analyze_bb_vinfo): Likewise.
	(aarch64_add_stmt_cost): Call aarch64_analyze_loop_vinfo or
	aarch64_analyze_bb_vinfo on the first use of a costs structure.
	Detect whether we're vectorizing a loop for SVE that might be
	completely unrolled if it used Advanced SIMD instead.
	(aarch64_adjust_body_cost_for_latency): New function.
	(aarch64_finish_cost): Call it.