With SLP now being a graph with shared nodes across instances we have
to make sure to compute the load permutation of nodes once, not
overwriting the result of earlier analysis.

2020-01-28  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/93428
	* tree-vect-slp.c (vect_build_slp_tree_2): Compute the load
	permutation when the load node is created.
	(vect_analyze_slp_instance): Re-use it here.

	* gcc.dg/torture/pr93428.c: New testcase.