Unreachable code check in partial functions needs improvement

Partial functions don't appear to be checked for unreachable code to the same extent as `match` expressions (the trivial case with a variable pattern is checked properly):

```scala
Some(1) match { 
  case Some(a) => 2
  case Some(b) => 3 // warning: "unreachable code"
}

Some(Some(1)) collect { 
  case Some(a) => 2
  case Some(b) => 3 // no warning
}
```