Bug when using lambda partial function

## Minimized code and output

```
$ dotr
Starting dotty REPL...
scala> val pf1: PartialFunction[AnyVal, Int] = { case n: Int => n }
1 |val pf1: PartialFunction[AnyVal, Int] = { case n: Int => n }
  |                                               ^
  |    this case is unreachable since type A1 and class Integer are unrelated

scala>
```

## Expectation
No error in this case
