Implemented the Word64Popcnt TurboFan operator for x64.

src/compiler/x64/instruction-selector-x64.cc

Index: src/compiler/x64/instruction-selector-x64.cc
diff --git a/src/compiler/x64/instruction-selector-x64.cc b/src/compiler/x64/instruction-selector-x64.cc
index e77b005ea2a0d41b085b4413f3d166c0e708194f..bb19db3d83bac31f8d91dc3cac958feb0b97de9c 100644
--- a/src/compiler/x64/instruction-selector-x64.cc
+++ b/src/compiler/x64/instruction-selector-x64.cc
@@ -602,6 +602,12 @@ void InstructionSelector::VisitWord32Popcnt(Node* node) {
 }
 
 
+void InstructionSelector::VisitWord64Popcnt(Node* node) {
+  X64OperandGenerator g(this);
+  Emit(kX64Popcnt, g.DefineAsRegister(node), g.Use(node->InputAt(0)));
+}
+
+
 void InstructionSelector::VisitInt32Add(Node* node) {
   X64OperandGenerator g(this);
 
@@ -1614,7 +1620,8 @@ InstructionSelector::SupportedMachineOperatorFlags() {
       MachineOperatorBuilder::kWord32ShiftIsSafe |
       MachineOperatorBuilder::kWord32Ctz | MachineOperatorBuilder::kWord64Ctz;
   if (CpuFeatures::IsSupported(POPCNT)) {
-    flags |= MachineOperatorBuilder::kWord32Popcnt;
+    flags |= MachineOperatorBuilder::kWord32Popcnt |
+             MachineOperatorBuilder::kWord64Popcnt;
   }
   if (CpuFeatures::IsSupported(SSE4_1)) {
     flags |= MachineOperatorBuilder::kFloat64RoundDown |