[LoopDataPrefetch] Make it testable with opt

Summary:
Since this is an IR pass it's nice to be able to write tests without
llc.  This is the counterpart of the llc test under
CodeGen/PowerPC/loop-data-prefetch.ll.

Reviewers: hfinkel

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D17464

llvm-svn: 261578