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.

llvm-svn: 161302
diff --git a/llvm/lib/Target/PowerPC/PPCInstr64Bit.td b/llvm/lib/Target/PowerPC/PPCInstr64Bit.td
index 91c5366..92a5051 100644
--- a/llvm/lib/Target/PowerPC/PPCInstr64Bit.td
+++ b/llvm/lib/Target/PowerPC/PPCInstr64Bit.td
@@ -265,6 +265,11 @@
              PPC970_DGroup_First, PPC970_Unit_FXU;
 }
 
+let Pattern = [(set G8RC:$rT, readcyclecounter)] in
+def MFTB8 : XFXForm_1_ext<31, 371, 268, (outs G8RC:$rT), (ins),
+                          "mftb $rT", SprMFTB>,
+            PPC970_DGroup_First, PPC970_Unit_FXU;
+
 let Defs = [X1], Uses = [X1] in
 def DYNALLOC8 : Pseudo<(outs G8RC:$result), (ins G8RC:$negsize, memri:$fpsi),"",
                        [(set G8RC:$result,