tree-optimization/103219 - avoid ICE in unroll-and-jam

For no particularly good reason unroll-and-jam uses single_dom_exit
to determine the exit for the region it wants to run VN on.  That
happens to ICE because of the dominance restriction.  Use single_exit
instead.

2021-11-15  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/103219
	* gimple-loop-jam.c (tree_loop_unroll_and_jam): Use single_exit
	to determine the exit for the VN region.

	* gcc.dg/torture/pr103219.c: New testcase.