tree-optimization/116818 - try VMAT_GATHER_SCATTER also for SLP

When not doing SLP and we end up with VMAT_ELEMENTWISE we consider
using strided loads, aka VMAT_GATHER_SCATTER.  The following moves
this logic down to also apply to SLP where we now can end up
using VMAT_ELEMENTWISE as well.

	PR tree-optimization/116818
	* tree-vect-stmts.cc (get_group_load_store_type): Consider
	VMAT_GATHER_SCATTER instead of VMAT_ELEMENTWISE also for SLP.
	(vectorizable_load): For single-lane VMAT_GATHER_SCATTER also
	ignore permutations.