[Bug tree-optimization/121764] constant_pointer_difference and ptr_difference_const seem to do the same thing

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121764

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note ptr_difference_const can handle some cases which
constant_pointer_difference  can't. Mostly dealing with &a[i] and &a[i+1] .

here is a C example that shows constant_pointer_difference is not handling this
case:
```
char a[1024];

__attribute__((noinline, noclone)) int
f1 (int q, int z, int t)
{
  __builtin_memcpy (&a[t], "abcd", 4);
  if (q)
    z = z + 123;
  else
    z *= 114;
  __builtin_memset (&a[t]+4, ' ', 2);
  return z;
}
```