tree-optimization/117041 - fix load classification of former grouped load

When we first detect a grouped load but later dis-associate it we
only set DR_GROUP_FIRST_ELEMENT to NULL, indicating it is not a
STMT_VINFO_GROUPED_ACCESS but leave DR_GROUP_NEXT_ELEMENT set.  This
causes a stray DR_GROUP_NEXT_ELEMENT access in get_group_load_store_type
to go wrong, indicating a load isn't single_element_p when it actually
is, leading to wrong classification and an ICE.

	PR tree-optimization/117041
	* tree-vect-stmts.cc (get_group_load_store_type): Only
	check DR_GROUP_NEXT_ELEMENT for STMT_VINFO_GROUPED_ACCESS.

	* gcc.dg/torture/pr117041.c: New testcase.