Inlining private method breaks compilation

## Compiler version

3.1.0

## Minimized code

```Scala
trait T {
  inline def foo(handler: Int): Unit =
    bar(handler)

  private inline def bar(handler: Int): Unit = ()
}

new T {
  foo(42)
}
```

## Output

```scala
undefined: T_this.bar # -1: TermRef(TermRef(NoPrefix,val T_this),bar) at inlining
```

## Expectation

Successful compilation
