The InlineSpiller was accessing the DominatorTreeBase directly through the public data member DT in the MachineDominatorTree. This is not a good idea as the "cached" information in SplitCriticalEdges is not applied before the access. The DominatorTreeBase must be accessed through the member function getBase() in MachineDominatorTree.
The fault was introduced in r266162.
I think the public data member DT in the MachineDominatorTree should have been made private in the original code (r215576) that introduced the concept of lazily updating the MachineDominatorTree information from