Don't share functional units among the PPC itineraries

Instead of sharing functional unit names between the various PPC itineraries,
give each core its own unit names prefixed with the core name.  This follows
the convention used by other backends (such as ARM), and removes a non-obvious
ordering dependency between the various PPCSchedule*.td files.

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195908 91177308-0d34-0410-b5e6-96231b3b80d8