tree-optimization/103458 - avoid creating new loops in CD-DCE

When creating forwarders in CD-DCE we have to avoid creating loops
where we formerly did not consider those because of abnormal
predecessors.  At this point simply excuse us when there are any
abnormal predecessors.

2021-11-29  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/103458
	* tree-ssa-dce.c (make_forwarders_with_degenerate_phis): Do not
	create forwarders for blocks with abnormal predecessors.

	* gcc.dg/torture/pr103458.c: New testcase.