SLPVectorizer: Add support for vectorizing trees that start at compare instructions.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179504 91177308-0d34-0410-b5e6-96231b3b80d8
Rename the slp-vectorizer clang/llvm flags. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179505 91177308-0d34-0410-b5e6-96231b3b80d8