Add readcyclecounter lowering on PPC64.

On PPC64, this can be done with a simple TableGen pattern.
To enable this, I've added the (otherwise missing) readcyclecounter
SDNode definition to TargetSelectionDAG.td.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161302 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/PowerPC/PPCISelLowering.cpp b/lib/Target/PowerPC/PPCISelLowering.cpp
index 13250b3..aa819ee 100644
--- a/lib/Target/PowerPC/PPCISelLowering.cpp
+++ b/lib/Target/PowerPC/PPCISelLowering.cpp
@@ -394,8 +394,10 @@
     setOperationAction(ISD::BUILD_VECTOR, MVT::v4f32, Custom);
   }
 
-  if (Subtarget->has64BitSupport())
+  if (Subtarget->has64BitSupport()) {
     setOperationAction(ISD::PREFETCH, MVT::Other, Legal);
+    setOperationAction(ISD::READCYCLECOUNTER, MVT::i64, Legal);
+  }
 
   setOperationAction(ISD::ATOMIC_LOAD,  MVT::i32, Expand);
   setOperationAction(ISD::ATOMIC_STORE, MVT::i32, Expand);