`Fill match arms` assist ignores existing variants prefixed with `Self::`

Right now if you have an enum that has some match arms and a match like this:

```rust
match x {
    Self::V1 => todo!(),
    Self::V2 => todo!(),
}
```

and you add a new variant and then run the fill match arms assist it will insert additional copies of `V1` and `V2` in the match arms that are prefixed with the types own name rather than Self.