fix list type unification (#17424)

fix list type unification (#17424)

Summary:
Previously we were unifying the types of lists across if block outputs. This now fails with Optional subtyping because two types which can be unified have different runtime representations.
```

            torch.jit.script
            def list_optional_fails(x):
                # type: (bool) -> Optional[int]
                if x:
                    y = [1]
                else:
                    y = [None]
                return y[0]

```
the indexing op will expect y to be a generic list, but it will find an intlist.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17424

Differential Revision: D14210903

Pulled By: eellison

fbshipit-source-id: 4b8b26ba2e7e5bebf617e40316475f91e9109cc2