Fix leak of SLP nodes when building store interleaving

The following fixes a leak of the discovered single-lane store
SLP nodes from which we only use their children.  This uncovers
a latent reference counting issue in the interleaving build where
we fail to increment their reference count.

	* tree-vect-slp.cc (vect_build_slp_store_interleaving):
	Fix reference counting.
	(vect_build_slp_instance): Release rhs_nodes.