Add some constantness in MachinePostDominators.h.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176784 91177308-0d34-0410-b5e6-96231b3b80d8
Remove unneeded #include.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176785 91177308-0d34-0410-b5e6-96231b3b80d8