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

src/mips64/assembler-mips64.h

Index: src/mips64/assembler-mips64.h
diff --git a/src/mips64/assembler-mips64.h b/src/mips64/assembler-mips64.h
index 8d9e935fd19d1d0ffba326b54d74329c088afcbd..f0f54aab1c509c122af24fe5ff51ece66abcb862 100644
--- a/src/mips64/assembler-mips64.h
+++ b/src/mips64/assembler-mips64.h
@@ -862,6 +862,7 @@ class Assembler : public AssemblerBase {
   void movn_d(FPURegister fd, FPURegister fs, Register rt);
   // Bit twiddling.
   void clz(Register rd, Register rs);
+  void dclz(Register rd, Register rs);
   void ins_(Register rt, Register rs, uint16_t pos, uint16_t size);
   void ext_(Register rt, Register rs, uint16_t pos, uint16_t size);
   void dext_(Register rt, Register rs, uint16_t pos, uint16_t size);