[TTI] Add getCacheLineSize

Summary:
And use it in PPCLoopDataPrefetch.cpp.

@hfinkel, please let me know if your preference would be to preserve the
ppc-loop-prefetch-cache-line option in order to be able to override the
value of TTI::getCacheLineSize for PPC.

Reviewers: hfinkel

Subscribers: hulx2000, mcrosier, mssimpso, hfinkel, llvm-commits

Differential Revision: http://reviews.llvm.org/D16306

llvm-svn: 258419