tree-optimization/116658 - latent issue in vect_is_slp_load_node

Permute nodes do not have a representative so we have to guard
vect_is_slp_load_node against those.

	PR tree-optimization/116658
	* tree-vect-slp.cc (vect_is_slp_load_node): Make sure
	node isn't a permute.

	* g++.dg/vect/pr116658.cc: New testcase.