[Bug middle-end/126887] backward threader cannot peel a single-iteration loop that the DOM threader peels

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126887

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
gcc.dg/uninit-1.c (bogus warning at any -O)

is not fixed.  Here DOM seems to (late) peel the first outer loop iteration,
another peeling shape we do not allow.

  for (p = el; p; p = p->next)
  {
    for (q = el; q != p; q = q->next)
  ...
    r = p;
  }

the inner loop guard q != p is trivially false, so r is initialized always.
Also uninit-1.c runs with -O1 where ranger based threading seems not to run
at all.

IIRC I tried to enable such theadings but had to revert
(r16-303-g1a13684dfc7286)