Implemented the Word64Clz TurboFan operator for x64, arm64, and mips64.

src/compiler/instruction-selector.cc

Index: src/compiler/instruction-selector.cc
diff --git a/src/compiler/instruction-selector.cc b/src/compiler/instruction-selector.cc
index c922f955d00b580d79200d0ac5a5a5c41ea6690b..64f57026a9d7db81c2573791144b7d6331eb80b9 100644
--- a/src/compiler/instruction-selector.cc
+++ b/src/compiler/instruction-selector.cc
@@ -736,6 +736,8 @@ void InstructionSelector::VisitNode(Node* node) {
       return MarkAsWord64(node), VisitWord64Sar(node);
     case IrOpcode::kWord64Ror:
       return MarkAsWord64(node), VisitWord64Ror(node);
+    case IrOpcode::kWord64Clz:
+      return MarkAsWord64(node), VisitWord64Clz(node);
     case IrOpcode::kWord64Equal:
       return VisitWord64Equal(node);
     case IrOpcode::kInt32Add:
@@ -971,6 +973,9 @@ void InstructionSelector::VisitWord64Sar(Node* node) { UNIMPLEMENTED(); }
 void InstructionSelector::VisitWord64Ror(Node* node) { UNIMPLEMENTED(); }
 
 
+void InstructionSelector::VisitWord64Clz(Node* node) { UNIMPLEMENTED(); }
+
+
 void InstructionSelector::VisitWord64Equal(Node* node) { UNIMPLEMENTED(); }