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?
