Remove the mixed stmt_vec_info/SLP node record_stmt_cost overload

Remove the mixed stmt_vec_info/SLP node record_stmt_cost overload

The following changes the record_stmt_cost calls in
vectorizable_load/store to only pass the SLP node when costing
vector stmts.  For now we'll still pass the stmt_vec_info,
determined from SLP_TREE_REPRESENTATIVE, so this merely cleans up
the API.

	* tree-vectorizer.h (record_stmt_cost): Remove mixed
	stmt_vec_info/SLP node inline overload.
	* tree-vect-stmts.cc (vectorizable_store): For costing
	vector stmts only pass SLP node to record_stmt_cost.
	(vectorizable_load): Likewise.