[PowerPC] Really iterate over all loops in PPCLoopDataPrefetch/PPCLoopPreIncPrep

[PowerPC] Really iterate over all loops in PPCLoopDataPrefetch/PPCLoopPreIncPrep

When I fixed these a couple of days ago to iterate over all loops, not just
depth == 1 loops, I inadvertently made it such that we'd only look at the first
top-level loop. Make sure that we really look at all of them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234705 91177308-0d34-0410-b5e6-96231b3b80d8