Add a comment about overlapping PPC frame offsets

I don't think that it is otherwise clear how the overlapping offsets
are processed into distinct spill slots. Comment that this is done
in processFunctionBeforeFrameFinalized.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177094 91177308-0d34-0410-b5e6-96231b3b80d8