Replacing root node of dominator tree

I'm trying to update a postdominator tree, but it seems using the DominatorTree family of classes I can't replace the root node of the tree. When the exit block is merged with one its predecessors I need to update the tree to use the new merged root and remove the old node.

The changeImmediateDominator functions reject a null replacement immediate dominator and I don't see any way of replacing the root short of rerunning the analysis on the whole function when I run into this case.

Do these need the addition of a setRootNode function to handle this case?