aarch64: Add costs for storing one element of a vector

aarch64: Add costs for storing one element of a vector

Storing one element of a vector is costed as a vec_to_scalar
followed by a scalar_store.  However, vec_to_scalar is also
used for reductions and for vector-to-GPR moves, which makes
it difficult to pick one cost for them all.

This patch therefore adds a cost for extracting one element
of a vector in preparation for storing it out.  The store
itself is still costed separately.

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

gcc/
	* config/aarch64/aarch64-protos.h
	(simd_vec_cost::store_elt_extra_cost): New member variable.
	* config/aarch64/aarch64.c (generic_advsimd_vector_cost): Update
	accordingly, using the vec_to_scalar cost for the new field.
	(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_detect_vector_stmt_subtype): Detect single-element stores.