### What it does
Checks for `a op= a op b` or `a op= b op a` patterns.

### Why is this bad?
Most likely these are bugs where one meant to write `a
op= b`.

### Known problems
Clippy cannot know for sure if `a op= a op b` should have
been `a = a op a op b` or `a = a op b`/`a op= b`. Therefore, it suggests both.
If `a op= a op b` is really the correct behavior it should be
written as `a = a op a op b` as it's less confusing.

### Example
```
let mut a = 5;
let b = 2;
// ...
a += a + b;
```