CTZ instruction implemented as optional operator.

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 516a9a7691ca0d7c2ba9c33a47857bc07dec48e3..3cff64be727f014e5f79ca990b345f077b080613 100644
--- a/src/compiler/x64/instruction-selector-x64.cc
+++ b/src/compiler/x64/instruction-selector-x64.cc
@@ -578,6 +578,12 @@ void InstructionSelector::VisitWord32Clz(Node* node) {
 }
 
 
+void InstructionSelector::VisitWord32Ctz(Node* node) {
+  X64OperandGenerator g(this);
+  Emit(kX64Tzcnt32, g.DefineAsRegister(node), g.Use(node->InputAt(0)));
+}
+
+
 void InstructionSelector::VisitInt32Add(Node* node) {
   X64OperandGenerator g(this);
 
@@ -1704,7 +1710,8 @@ InstructionSelector::SupportedMachineOperatorFlags() {
       MachineOperatorBuilder::kFloat32Min |
       MachineOperatorBuilder::kFloat64Max |
       MachineOperatorBuilder::kFloat64Min |
-      MachineOperatorBuilder::kWord32ShiftIsSafe;
+      MachineOperatorBuilder::kWord32ShiftIsSafe |
+      MachineOperatorBuilder::kWord32Ctz;
   if (CpuFeatures::IsSupported(SSE4_1)) {
     flags |= MachineOperatorBuilder::kFloat64RoundDown |
              MachineOperatorBuilder::kFloat64RoundTruncate;