[Bug tree-optimization/126762] missing usatadd due to pre

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126762

--- Comment #1 from Drea Pinski <pinskia at gcc dot gnu.org> ---
That is we get:
```
  _7 = .ADD_OVERFLOW (a_5(D), cst_6(D));
  _2 = IMAGPART_EXPR <_7>;
  if (_2 != 0)
    goto <bb 4>; [50.00%]
  else
    goto <bb 3>; [50.00%]

  <bb 3> [local count: 536870912]:
  _1 = REALPART_EXPR <_7>;
  _10 = _1 + 4294967292;

  <bb 4> [local count: 1073741824]:
  # prephitmp_11 = PHI <_10(3), 4294967291(2)>
```

Instead of figuring out -4294967292 + 4294967291 = -1u.