tree-optimization/100492 - avoid irreducible regions in loop distribution

tree-optimization/100492 - avoid irreducible regions in loop distribution

When we distribute away a condition we rely on the ability to
change it to either 1 != 0 or 0 != 0 depending on the direction
of the exit branch in the respective loop.  But when the loop
contains an irreducible sub-region then for the conditions inside
this this fails and can lead to infinite loops being generated.

Avoid distibuting loops with irreducible sub-regions.

2021-05-10  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/100492
	* tree-loop-distribution.c (find_seed_stmts_for_distribution):
	Find nothing when the loop contains an irreducible region.

	* gcc.dg/torture/pr100492.c: New testcase.