if/alt expressions don't understand how to return functions that fail

In the following example the first arm of the if expression doesn't translate:

```
fn f() -> ! { fail }

fn g() -> int {
  auto x = if (true) {
    f()
  } else {
    10
  };
  ret x;
}
```

Trans thinks that f() returns int (because x is int) and tries to copy the result of f, which is not int. Replacing f() with a plain fail expression works as expected.
