[LCG] Rather than walking the directed graph structure to update graph

[LCG] Rather than walking the directed graph structure to update graph
pointers in node objects, just walk the map from function to node.

It doesn't have stable ordering, but works just as well and is much
simpler. We don't need ordering when just updating internal pointers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310163 91177308-0d34-0410-b5e6-96231b3b80d8