tree-optimization/101026 - fix SLP re-association

Since we cannot yet encode the operation in the SLP node itself
but need a representative stmt require an existing one for now
to avoid the need to build a fake GIMPLE stmt.

2021-06-11  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/101026
	* tree-vect-slp.c (vect_build_slp_tree_2): Make sure we
	have a representative for the associated chain nodes.

	* gfortran.dg/pr101026.f: New testcase.