Add diagnostics to the vectorizer cost model.

When the cost model determines vectorization is not possible/profitable these remarks print an analysis of that decision.

Note that in selectVectorizationFactor() we can assume that OptForSize and ForceVectorization are mutually exclusive.

Reviewed by Arnold Schwaighofer


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214599 91177308-0d34-0410-b5e6-96231b3b80d8