[LoopDataPrefetch] Add TTI to limit the number of iterations to prefetch ahead

[LoopDataPrefetch] Add TTI to limit the number of iterations to prefetch ahead

Summary:
It can hurt performance to prefetch ahead too much.  Be conservative for
now and don't prefetch ahead more than 3 iterations on Cyclone.

Reviewers: hfinkel

Subscribers: llvm-commits, mzolotukhin

Differential Revision: http://reviews.llvm.org/D17949

llvm-svn: 263772