reenable r101565, removing a problematic assertion.

CGSCC can delete nodes in regions of the callgraph that
have already been visited.  If new CG nodes are allocated
to the same pointer, we shouldn't abort, just handle it
correctly by assigning a new number.  This should restore
stability by removing invalidated pointers that *will* be
reused from the densemap in the iterator.

llvm-svn: 101628