SLP Vectorizer: Fix a bug in the code that does CSE on the generated gather sequences.
Make sure that we don't replace and RAUW two sequences if one does not dominate the other.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184674 91177308-0d34-0410-b5e6-96231b3b80d8