[Bug tree-optimization/23970] loop-invariant-motion is not doing it's work

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=23970

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Unswitching does this as a separate transform now, the "hoist guards"
transform.
It's even done completely separate now:

unsigned int
tree_ssa_unswitch_loops (function *fun)
{   
  bool changed_unswitch = false;
  bool changed_hoist = false;
  auto_edge_flag ignored_edge_flag (fun);

  ranger = enable_ranger (fun);

  /* Go through all loops starting from innermost, hoisting guards.  */
  for (auto loop : loops_list (fun, LI_FROM_INNERMOST))
    { 
      if (loop->inner)
        changed_hoist |= tree_unswitch_outer_loop (loop);
    }

  /* Go through innermost loops, unswitching on invariant predicates
     within those.  */
...

the question is how we'd call the "cheap" unswitching transform.  I guess
simply testing optimize > 2 || opt_enabled or so isn't quite desirable.

Note it's possible to separate this into an entirely separate pass as well.