[PowerPC] Add a flag for experimenting with subreg liveness tracking

This cannot yet be enabled by default, it causes ~50 miscompiles in the test
suite.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225497 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/PowerPC/PPCSubtarget.h

@@ -259,6 +259,8 @@ public:
                            MachineInstr *end,
                            unsigned NumRegionInstrs) const override;
   bool useAA() const override;
+
+  bool enableSubRegLiveness() const override;
 };
 } // End llvm namespace