[cfgloop] PR middle-end/68375: Restructure get_loop_body_in_bfs_order to handle loops with only a header

	PR middle-end/68375
	* cfgloop.c (get_loop_body_in_bfs_order): Restructure loop to avoid
	bogus assertion.

	* gcc.dg/pr68375.c: New test.

From-SVN: r230798