[LoopUnroll] Pass SCEV to getUnrollingPreferences hook. NFCI.

Reviewers: sanjoy, anna, reames, apilipenko, igor-laevsky, mkuper

Subscribers: jholewinski, arsenm, mzolotukhin, nemanjai, nhaehnle, javed.absar, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D34531

llvm-svn: 306554