LoopVectorizer: When -Os is used, vectorize only loops that dont require a...

LoopVectorizer: When -Os is used, vectorize only loops that dont require a tail loop. There is no testcase because I dont know of a way to initialize the loop vectorizer pass without adding an additional hidden flag. 

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169950 91177308-0d34-0410-b5e6-96231b3b80d8