This fixes a bug when an original property (`obj.i`) shares the same
name as a mangled property (`obj.prop` -> `n.i`), and you're using a
name-cache. Because our mangled name collides, we have to ensure all
original `.i` properties are renamed to something else (`n.o`).

But, because we had a name-cache, we would reserve the mangled property
name in the mangler. That prevents us from attempting to rename the
original `.i` property to `.o`.