[LLVMdev] Replacing root node of dominator tree

[Matt Arsenault]

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?