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
