Don't model the fetch and decode units for the PPC440

Modeling the fetch and decode units in the PPC440 itinerary does not add
anything to the hazard detection capability (and so modeling them just wastes
compile time).

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195946 91177308-0d34-0410-b5e6-96231b3b80d8