tree-optimization/117709 - bogus offset for gather load

When diverting to VMAT_GATHER_SCATTER we fail to zero *poffset
which was previously set if a load was classified as
VMAT_CONTIGUOUS_REVERSE.  The following refactors
get_group_load_store_type a bit to avoid this but this all needs
some serious TLC.

	PR tree-optimization/117709
	* tree-vect-stmts.cc (get_group_load_store_type): Only
	set *poffset when we end up with VMAT_CONTIGUOUS_DOWN
	or VMAT_CONTIGUOUS_REVERSE.