Support making a variable Optional in an else branch (#11002)
That is, support patterns such as:
```
if condition:
foo = Foo()
else:
foo = None
```
Currently this does not work, but the the *reverse* does (because foo
will be inferred as a PartialType).
I think it might be worth tackling this in a more general way, for
other types, though I think that is a little fiddlier and likely to be
more controversial, so I'm starting with something special-cased for
the "assigning literal None" case first.
The rule we implement is that we allow updating the type of a variable
when assigning `None` to it if the variable's type was inferred and it
was defined in an earlier branch of the same `if/then/else` statement.
Some infrastructure is added to make determinations about that.
