Don't wrap the entry map for LiveInfo in Maybe.

It never really encoded a invariant.

* The linear register allocator just did partial pattern matches
* The graph allocator just set it to (Just mapEmpty) for Nothing

So I changed LiveInfo to directly contain the map.

Further natCmmTopToLive which filled in Nothing is no longer exported.
Instead we know call cmmTopLiveness which changes the type AND fills
in the map.