Fix memory leak in vect_analyze_loop_form

Fix memory leak in vect_analyze_loop_form

The following fixes a memory leak in vect_analyze_loop_form which fails
to free the loop body it gets.  It also allows more countable exits,
matching what we can handle later, when we decide which exit to use
as main exit.  Finally some no longer applying comments are adjusted.

	* tree-vect-loop.cc (vec_init_loop_exit_info): Adjust comment,
	prefer all later exits we can handle.
	(vect_analyze_loop_form): Free the allocated loop body.
	Adjust comments.