The CodeCleaner treated all expressions with type `Nothing` as
non-terminating. This is not true for cases when return type is generic,
e.g:
```
fun <T> foo(): T = Any() as T
foo<Nothing>() // type: Nothing
```
This change makes the CodeCleaner more conservative, so that it doesn't
treat to such functions as non-terminating. This eliminates the need
to change call types from Nothing to Unit in
KotlinNothingValueExceptionLowering.
Anton Bannykh
b5f31a3d76