Add debug_vn_reference_ops helper

This factors out a helper to dump VN reference operands, sth that
proves useful in debugging VN issues.

2021-04-07  Richard Biener  <rguenther@suse.de>

	* tree-ssa-sccvn.h (print_vn_reference_ops): Declare.
	* tree-ssa-pre.c (print_pre_expr): Factor out VN reference operand
	printing...
	* tree-ssa-sccvn.c (print_vn_reference_ops): ... into this new
	function.
	(debug_vn_reference_ops): New.