MIR-borrowck: augmented assignment causes duplicate errors

[arielb1]

Augmented assignments include both a read and a write, so naturally when there's a conflict they cause 2 borrowck errors:

For example, this test (borrowck-assign-to-andmut-in-borrowed-loc)

struct S<'a> {
    pointer: &'a mut isize
}

fn copy_borrowed_ptr<'a>(p: &'a mut S<'a>) -> S<'a> {
    S { pointer: &mut *p.pointer }
}

fn main() {
    let mut x = 1;

    {
        let mut y = S { pointer: &mut x };
        let z = copy_borrowed_ptr(&mut y);
        *y.pointer += 1; //~ ERROR cannot assign
        *z.pointer += 1;
    }
}

Triggers a duplicate error:

error[E0506]: cannot assign to `*y.pointer` because it is borrowed (Ast)
  --> ../src/test/compile-fail/borrowck/borrowck-assign-to-andmut-in-borrowed-loc.rs:28:9
   |
27 |         let z = copy_borrowed_ptr(&mut y);
   |                                        - borrow of `*y.pointer` occurs here
28 |         *y.pointer += 1; //~ ERROR cannot assign
   |         ^^^^^^^^^^^^^^^ assignment to borrowed `*y.pointer` occurs here

error[E0503]: cannot use `(*y.pointer)` because it was mutably borrowed (Mir)
  --> ../src/test/compile-fail/borrowck/borrowck-assign-to-andmut-in-borrowed-loc.rs:28:9
   |
27 |         let z = copy_borrowed_ptr(&mut y);
   |                                   ------ borrow of `y` occurs here
28 |         *y.pointer += 1; //~ ERROR cannot assign
   |         ^^^^^^^^^^^^^^^ use of borrowed `y`

error[E0506]: cannot assign to `(*y.pointer)` because it is borrowed (Mir)
  --> ../src/test/compile-fail/borrowck/borrowck-assign-to-andmut-in-borrowed-loc.rs:28:9
   |
27 |         let z = copy_borrowed_ptr(&mut y);
   |                                   ------ borrow of `(*y.pointer)` occurs here
28 |         *y.pointer += 1; //~ ERROR cannot assign
   |         ^^^^^^^^^^^^^^^ assignment to borrowed `(*y.pointer)` occurs here

error: aborting due to 3 previous errors

Not sure what's the best way to solve this, but this issue alone is fairly low priority.

[davidtwco]

I'll tackle this one.