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

test/unittests/compiler/mips64/instruction-selector-mips64-unittest.cc

Index: test/unittests/compiler/mips64/instruction-selector-mips64-unittest.cc
diff --git a/test/unittests/compiler/mips64/instruction-selector-mips64-unittest.cc b/test/unittests/compiler/mips64/instruction-selector-mips64-unittest.cc
index e3548f80969de8dd0260c106024fa333c5f91131..148667333bcbc2677e05f25cc698c98c4c343890 100644
--- a/test/unittests/compiler/mips64/instruction-selector-mips64-unittest.cc
+++ b/test/unittests/compiler/mips64/instruction-selector-mips64-unittest.cc
@@ -841,6 +841,21 @@ TEST_F(InstructionSelectorTest, Word32Clz) {
 }
 
 
+TEST_F(InstructionSelectorTest, Word64Clz) {
+  StreamBuilder m(this, kMachUint64, kMachUint64);
+  Node* const p0 = m.Parameter(0);
+  Node* const n = m.Word64Clz(p0);
+  m.Return(n);
+  Stream s = m.Build();
+  ASSERT_EQ(1U, s.size());
+  EXPECT_EQ(kMips64Dclz, s[0]->arch_opcode());
+  ASSERT_EQ(1U, s[0]->InputCount());
+  EXPECT_EQ(s.ToVreg(p0), s.ToVreg(s[0]->InputAt(0)));
+  ASSERT_EQ(1U, s[0]->OutputCount());
+  EXPECT_EQ(s.ToVreg(n), s.ToVreg(s[0]->Output()));
+}
+
+
 TEST_F(InstructionSelectorTest, Float32Abs) {
   StreamBuilder m(this, kMachFloat32, kMachFloat32);
   Node* const p0 = m.Parameter(0);