dump SLP_TREE_REPRESENTATIVE

It always annoyed me to see those empty SLP nodes in dumpfiles:

t.c:16:3: note:   node 0x3a2a280 (max_nunits=1, refcnt=1)
t.c:16:3: note:         { }
t.c:16:3: note:         children 0x3a29db0 0x3a29e90

resulting from two-operator handling.  The following makes
sure to also dump the operation template or VEC_PERM_EXPR.

2020-11-20  Richard Biener  <rguenther@suse.de>

	* tree-vect-slp.c (vect_print_slp_tree): Also dump
	SLP_TREE_REPRESENTATIVE.