AArch64: re-enable memory access costing after SLP change.

While chasing down a costing difference between SLP and non-SLP for memory
access costing I noticed that at some point the SLP and non-SLP costing have
diverged.  It used to be we only supported LOAD_LANES in SLP and so the non-SLP
costing was working fine.

But with the change to SLP only we now lost costing.

It looks like the vectorizer for non-SLP stores the VMAT type in
STMT_VINFO_MEMORY_ACCESS_TYPE on the stmt_info, but for SLP it stores it in
SLP_TREE_MEMORY_ACCESS_TYPE which is on the SLP node itself.

While my first attempt of a patch was to just also store the VMAT in the
stmt_info https://gcc.gnu.org/pipermail/gcc-patches/2024-October/665295.html
Richi pointed out that this goes wrong when the same access is used Hybrid.

And so we have to do a backend specific fix.  To help out other backends this
also introduces a generic helper function suggested by Richi in that patch
(I hope that's ok.. I didn't want to split out just the helper.)

This successfully restores VMAT based costing in the new SLP only world.

gcc/ChangeLog:

	* tree-vectorizer.h (vect_mem_access_type): New.
	* config/aarch64/aarch64.cc (aarch64_ld234_st234_vectors): Use it.
	(aarch64_detect_vector_stmt_subtype): Likewise.
	(aarch64_adjust_stmt_cost): Likewise.
	(aarch64_vector_costs::count_ops): Likewise.
	(aarch64_vector_costs::add_stmt_cost): Make SLP node named.