[Bug tree-optimization/81455] [7/8 Regression] Compile-time hog w/ -O1 -funswitch-loops

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81455

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Ok, so we end up running in circles in find_loop_guard

  gcond *cond;
  do
    {
      if (single_succ_p (header))
        header = single_succ (header);
      else
        {
          cond = dyn_cast <gcond *> (last_stmt (header));
          if (! cond)
            return NULL;
          extract_true_false_edges_from_block (header, &te, &fe);
          /* Make sure to skip earlier hoisted guards that are left
             in place as if (true).  */
          if (gimple_cond_true_p (cond))
            header = te->dest;
          else if (gimple_cond_false_p (cond))
            header = fe->dest;
          else
            break;
        }
    }
  while (1);

for

  <bb 4> [85.00%] [count: INV] [loop 3 header]:
  if (1 != 0)
    goto <bb 9>; [85.00%] [count: INV]
  else
    goto <bb 11>; [15.00%] [count: INV]

  <bb 11> [12.75%] [count: INV]:
  goto <bb 5>; [100.00%] [count: INV]

  <bb 9> [72.25%] [count: INV] [loop 3 latch]:
  goto <bb 4>; [100.00%] [count: INV]

where we walk a latch.  We never should do that.