aarch64: Add reduction costs to simd_vec_costs

aarch64: Add reduction costs to simd_vec_costs

This patch is part of a series that makes opt-in tweaks to the
AArch64 vector cost model.

At the moment, all reductions are costed as vec_to_scalar, which
also includes things like extracting a single element from a vector.
This is a bit too coarse in practice, since the cost of a reduction
depends very much on the type of value that it's processing.
This patch therefore adds separate costs for each case.  To start with,
all the new costs are copied from the associated vec_to_scalar ones.

Due the extreme lateness of this patch in the GCC 11 cycle, I've added
a new tuning flag (use_new_vector_costs) that selects the new behaviour.
This should help to ensure that the risk of the new code is only borne
by the CPUs that need it.  Generic tuning is not affected.

gcc/
	* config/aarch64/aarch64-tuning-flags.def (use_new_vector_costs):
	New tuning flag.
	* config/aarch64/aarch64-protos.h (simd_vec_cost): Put comments
	above the fields rather than to the right.
	(simd_vec_cost::reduc_i8_cost): New member variable.
	(simd_vec_cost::reduc_i16_cost): Likewise.
	(simd_vec_cost::reduc_i32_cost): Likewise.
	(simd_vec_cost::reduc_i64_cost): Likewise.
	(simd_vec_cost::reduc_f16_cost): Likewise.
	(simd_vec_cost::reduc_f32_cost): Likewise.
	(simd_vec_cost::reduc_f64_cost): Likewise.
	* config/aarch64/aarch64.c (generic_advsimd_vector_cost): Update
	accordingly, using the vec_to_scalar_cost for the new fields.
	(generic_sve_vector_cost, a64fx_advsimd_vector_cost): Likewise.
	(a64fx_sve_vector_cost, qdf24xx_advsimd_vector_cost): Likewise.
	(thunderx_advsimd_vector_cost, tsv110_advsimd_vector_cost): Likewise.
	(cortexa57_advsimd_vector_cost, exynosm1_advsimd_vector_cost)
	(xgene1_advsimd_vector_cost, thunderx2t99_advsimd_vector_cost)
	(thunderx3t110_advsimd_vector_cost): Likewise.
	(aarch64_use_new_vector_costs_p): New function.
	(aarch64_simd_vec_costs): New function, split out from...
	(aarch64_builtin_vectorization_cost): ...here.
	(aarch64_is_reduction): New function.
	(aarch64_detect_vector_stmt_subtype): Likewise.
	(aarch64_add_stmt_cost): Call aarch64_detect_vector_stmt_subtype if
	using the new vector costs.