Borrow checker is not flow sensitive

[nikomatsakis]

This results in spurious errors. Proper fix is a bit tricky so as to avoid re-encoding the control flow that liveness/trans already know about. May come about as part of a refactoring I want to do to liveness to accommodate once fns.

[nikomatsakis]

Now with INHTWAMA this is more important. Some routines, such as these two from hashmap, cannot be checked without flow-sensitivity:

    fn find_or_insert(&mut self, k: K, v: V) -> &self/V {...}
    fn find_or_insert_with(&mut self, k: K, f: fn(&K) -> V) -> &self/V {...}

[nikomatsakis]

Part of #5074

[nikomatsakis]

Going to re-open and convert #5074 into a "meta" issue