unswitching of outer loops

This allows loop unswitching to unswitch outer loops conditions are
invariant in.  We restrict ourselves to unswitch conditions in innermost
loops and will only unswitch loop nests that do not contain any sibling loops.
To simplify the implementation the loop nest unswitched is the deepest all
unswitching candidates are invariant in.

For 507.cactuBSSN_r it can be observed we unswitch the outer loops
of the compute kernels for the fdOrder parameter.  It seems to be within
the existing growth limitations to perform the unswitchings, a performance
benefit is not seen.

	* tree-ssa-loop-unswitch.cc (init_loop_unswitch_info): First collect
	candidates and determine the outermost loop to unswitch.
	(tree_ssa_unswitch_loops): First perform all guard hoisting,
	then perform unswitching on innermost loop predicates.
	(find_unswitching_predicates_for_bb): Keep track of the
	outermost loop to unswitch.
	(evaluate_bbs): Adjust exit test.
	(tree_unswitch_single_loop): Dump whether we unswitched an outer
	loop.
	(tree_unswitch_loop): Remove assert we unswitch only innermost
	loops.

	* gcc.dg/loop-unswitch-18.c: New testcase.
	* gcc.dg/tree-ssa/loopclosedphi.c: Disable unswitching,
	adjust expected counts.
	* gcc.dg/torture/pr71462.c: Add -w to ignore undefined
	behavior diagnostics after now unswitching outer loops.