tree-optimization/117720 - check alignment for VMAT_STRIDED_SLP

While vectorizable_store was already checking alignment requirement
of the stores and fall back to elementwise accesses if not honored
the vectorizable_load path wasn't doing this.  After the previous
change to disregard alignment checking for VMAT_STRIDED_SLP in
get_group_load_store_type this now tripped on power.

	PR tree-optimization/117720
	* tree-vect-stmts.cc (vectorizable_load): For VMAT_STRIDED_SLP
	verify the choosen load type is OK with regard to alignment.