LoopVectorizer: Handle strided memory accesses by versioning

 for (i = 0; i < N; ++i)
   A[i * Stride1] += B[i * Stride2];

We take loops like this and check that the symbolic strides 'Strided1/2' are one
and drop to the scalar loop if they are not.

This is currently disabled by default and hidden behind the flag
'enable-mem-access-versioning'.

radar://13075509

llvm-svn: 198950