Also lower SLP grouped loads with just one consumer

This makes sure to produce interleaving schemes or load-lanes
for single-element interleaving and other permutes that otherwise
would use more than three vectors.

It exposes the latent issue that single-element interleaving with
large gaps can be inefficient - the mitigation in get_group_load_store_type
doesn't trigger when we clear the load permutation.

It also exposes the fact that not all permutes can be lowered in
the best way in a vector length agnostic way so I've added an
exception to keep power-of-two size contiguous aligned chunks
unlowered (unless we want load-lanes).  The optimal handling
of load/store vectorization is going to continue to be a learning
process.

	* tree-vect-slp.cc (vect_lower_load_permutations): Also
	process single-use grouped loads.
	Avoid lowering contiguous aligned power-of-two sized
	chunks, those are better handled by the vector size
	specific SLP code generation.
	* tree-vect-stmts.cc (get_group_load_store_type): Drop
	the unrelated requirement of a load permutation for the
	single-element interleaving limit.

	* gcc.dg/vect/slp-46.c: Remove XFAIL.