[Bug tree-optimization/114999] A few missing optimizations due to `a - b` and `b - a` not being detected as negatives of each other

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114999

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #2)
> Another one:
> ```
> int f(int a, int b, int x)
> {
>   x = a - b;
>   int y = -x;
>   return (x >= 0 ? x : 0) + (x <= 0 ? y : 0);
> }
> ```
> 
> This should be detected as `abs<a - b>` (just like if the assignment to x is
> removed.

```
int f(int a, int b, int x)
{
  x = a - b;
  int y = -x;
  return (x >= 0 ? x : 0) + (y >= 0 ? y : 0);
}
```