PPC: Add a better comment about the i64 FI fixup

In discussing this change with Bill Schmidt, it was decided that the original
comment about negative FIs was incorrect. We'll still exclude them for now, but
now with a more-accurate explanation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186005 91177308-0d34-0410-b5e6-96231b3b80d8