Enable the P8Model entry

This was missed last time around, for the P8 Instruction Scheduling
changes (223257). This will hook the P8Model entry in so those
changes will actually be used.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224452 91177308-0d34-0410-b5e6-96231b3b80d8