[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.

llvm-svn: 225497
diff --git a/llvm/lib/Target/PowerPC/PPCSubtarget.h b/llvm/lib/Target/PowerPC/PPCSubtarget.h
index de5f92a..b33b737 100644
--- a/llvm/lib/Target/PowerPC/PPCSubtarget.h
+++ b/llvm/lib/Target/PowerPC/PPCSubtarget.h
@@ -259,6 +259,8 @@
                            MachineInstr *end,
                            unsigned NumRegionInstrs) const override;
   bool useAA() const override;
+
+  bool enableSubRegLiveness() const override;
 };
 } // End llvm namespace