Fix RefreshCallGraph to use CGN->replaceCallEdge instead of hand
rolling its own loop. replaceCallEdge properly maintains the
reference counts of the nodes, fixing a crash exposed by the
iterative callgraph stuff.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@102120 91177308-0d34-0410-b5e6-96231b3b80d8