Fix unchecked uses of DominatorTree in MemoryDependenceAnalysis.

Use unknown results for places where it would be needed

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181176 91177308-0d34-0410-b5e6-96231b3b80d8
Revert r164763 because it introduces new shuffles.
Thanks Nick Lewycky for pointing this out.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181177 91177308-0d34-0410-b5e6-96231b3b80d8