Fix compound assignment type casts

[joaotgouveia]

Fixes type casts inside compound assignments under the unsafe model and removes unnecessary casts under the refcount model. When translating binary operators, both unsafe and refcount have a special case for translating compound assignments involving different types.

The types being compared were the original C++ types. However, differences between the original C++ types do not necessarily mean that the corresponding Rust types differ. This caused unnecessary casts to be introduced and, under the unsafe model, resulted in translations that do not compile, such as the following:

int main() {
  std::vector<int> v;
  v.push_back(10);
  v.front() += 5;
  assert(v.front() == 15);
  return v.front();
}

pub fn main() {
    unsafe {
        std::process::exit(main_0() as i32);
    }
}
unsafe fn main_0() -> i32 {
    let mut v: Vec<i32> = Vec::new();
    v.push(10);
    ((v).first_mut().unwrap()) = ((((v).first_mut().unwrap()) as i32) + 5) as i32; // invalid cast
    assert!(((*((v).first_mut().unwrap())) == (15)));
    return (*((v).first_mut().unwrap()));
}