LoopVectorizer: Enable strided memory accesses versioning per default

I saw no compile or execution time regressions on x86_64 -mavx -O3.

radar://13075509

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199015 91177308-0d34-0410-b5e6-96231b3b80d8