[Bug tree-optimization/53806] Missed optimization (a<=b)&&(a>=b) with trapping

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53806

--- Comment #4 from Drea Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Drea Pinski from comment #3)
> So this looks like this is optimized due to frange work.
> That is dom and evrp change `a<=b` into `a==b` and then ifcombine/phiopt can
> handle that.

well not exactly because we don't combine then until reassociation because of:

```
        /* If we changed the conditions that cause a trap, we lose.  */
        if ((ltrap || rtrap) != trap)
          return NULL_TREE;
```

Which I think this should be if we had didn't have a trap, don't introduce a
trap.

So this should be:
```
if (!ltrap && !rtrap && trap)
  return NULL_TREE;
```

Let me try that; that will allow phiopt to optimize it without reassociation.
(with the fix for PR 106164).

As far without the transformation of `a <= b` into `a == b`; I have an idea on
how to fix that and will be implementing it. But maybe the above change will
fix it too because we had 2 that traped before and none now.