Loop optimizers expect to be able to insert on the preheader
edge w/o splitting it thus avoid ending up with a preheader
that enters the loop via an EH edge (or an abnormal edge).

2020-05-06  Richard Biener  <rguenther@suse.de>

	PR middle-end/94964
	* cfgloopmanip.c (create_preheader): Require non-complex
	preheader edge for CP_SIMPLE_PREHEADERS.