refactor tryToVectorizePair to a new method that supports vectorization of lists.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179932 91177308-0d34-0410-b5e6-96231b3b80d8
SLPVectorizer: Reduce the compile time by eliminating the search for some of the more expensive patterns. After this change will only check basic arithmetic trees that start at cmpinstr.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179933 91177308-0d34-0410-b5e6-96231b3b80d8