Unloop loops that no longer loops in tree-ssa-loop-ch

I noticed this after adding sanity check that the upper bound on number
of iterations never drop to -1.  It seems to be relatively common case
(happening few hundred times in testsuite and also during bootstrap)
that loop-ch duplicates enough so the loop itself no longer loops.

This is later detected in loop unrolling but since we test the number
of iterations anyway it seems better to do that earlier.

	* cfgloopmanip.h (unloop_loops): Export.
	* tree-ssa-loop-ch.cc (ch_base::copy_headers): Unloop loops
	that no longer loop.
	* tree-ssa-loop-ivcanon.cc (unloop_loops): Export; do not free
	vectors of loops to unloop.
	(canonicalize_induction_variables): Free vectors here.
	(tree_unroll_loops_completely): Free vectors here.