middle-end: Pass along SLP node when costing vector loads/stores

With the support to SLP only we now pass the VMAT through the SLP node, however
the majority of the costing calls inside vectorizable_load and
vectorizable_store do no pass the SLP node along.  Due to this the backend costing
never sees the VMAT for these cases anymore.

Additionally the helper around record_stmt_cost when both SLP and stmt_vinfo are
passed would only pass the SLP node along.  However the SLP node doesn't contain
all the info available in the stmt_vinfo and we'd have to go through the
SLP_TREE_REPRESENTATIVE anyway.  As such I changed the function to just Always
pass both along.  Unlike the VMAT changes, I don't believe there to be a
correctness issue here but would minimize the number of churn in the backend
costing until vectorizer costing as a whole is revisited in GCC 16.

These changes re-enable the cost model on AArch64 and also correctly find the
VMATs on loads and stores fixing testcases such as sve_iters_low_2.c.

gcc/ChangeLog:

	* tree-vect-data-refs.cc (vect_get_data_access_cost): Pass NULL for SLP
	node.
	* tree-vect-stmts.cc (record_stmt_cost): Expose.
	(vect_get_store_cost, vect_get_load_cost): Extend with SLP node.
	(vectorizable_store, vectorizable_load): Pass SLP node to all costing.
	* tree-vectorizer.h (record_stmt_cost): Always pass both SLP node and
	stmt_vinfo to costing.
	(vect_get_load_cost, vect_get_store_cost): Extend with SLP node.