[LoopInterchange] Make isProfitableForVectorization slightly more conservative.

After D43236, we started interchanging loops with empty dependence
matrices.  In isProfitableForVectorization, we try to determine if
interchanging makes the loop dependences more friendly to the
vectorizer. If there are no dependences, we should not interchange,
based on that heuristic.

Reviewers: efriedma, mcrosier, karthikthecool, blitz.opensource

Reviewed By: mcrosier

Differential Revision: https://reviews.llvm.org/D45208

llvm-svn: 330738