PPC: Add a FIXME regarding the non-working fma+fneg Altivec pattern

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178658 91177308-0d34-0410-b5e6-96231b3b80d8
PPC: Enable FRES and FRSQRTE on the default PPC64 description

I discussed this with Bill Schmidt on IRC, and it was decided that this is a
safe and reasonable default.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178659 91177308-0d34-0410-b5e6-96231b3b80d8