Completely merge the implementation details of DomTree and PostDomTree.

Also, add a FIXME for a bug in PostDomTree calculation I noticed while writing this,


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@42593 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed