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

src/compiler/mips64/instruction-selector-mips64.cc

Index: src/compiler/mips64/instruction-selector-mips64.cc
diff --git a/src/compiler/mips64/instruction-selector-mips64.cc b/src/compiler/mips64/instruction-selector-mips64.cc
index 1b7066eb11daaa917786bd1b46c699704c14ba31..3ab119e6e2afb6bc35aacd0bb4d60f63c785c843 100644
--- a/src/compiler/mips64/instruction-selector-mips64.cc
+++ b/src/compiler/mips64/instruction-selector-mips64.cc
@@ -332,6 +332,11 @@ void InstructionSelector::VisitWord64Ror(Node* node) {
 }
 
 
+void InstructionSelector::VisitWord64Clz(Node* node) {
+  VisitRR(this, kMips64Dclz, node);
+}
+
+
 void InstructionSelector::VisitInt32Add(Node* node) {
   Mips64OperandGenerator g(this);
   // TODO(plind): Consider multiply & add optimization from arm port.