Use SparseSet for the RAFast live virtual register map.

This makes RAFast 4% faster, and it gets rid of the dodgy DenseMap
iteration.

This also revealed that RAFast would sometimes dereference DenseMap
iterators after erasing other elements from the map. That does seem to
work in the current DenseMap implementation, but SparseSet doesn't allow
it.

llvm-svn: 151111