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

src/x64/assembler-x64.cc

Index: src/x64/assembler-x64.cc
diff --git a/src/x64/assembler-x64.cc b/src/x64/assembler-x64.cc
index b051a43f4dc00a42e3215a7f7025052cc5fdd92b..0c5ba9dd06ea5ea9beff7e7cb550f6bbaad930be 100644
--- a/src/x64/assembler-x64.cc
+++ b/src/x64/assembler-x64.cc
@@ -745,6 +745,24 @@ void Assembler::bsrl(Register dst, const Operand& src) {
 }
 
 
+void Assembler::bsrq(Register dst, Register src) {
+  EnsureSpace ensure_space(this);
+  emit_rex_64(dst, src);
+  emit(0x0F);
+  emit(0xBD);
+  emit_modrm(dst, src);
+}
+
+
+void Assembler::bsrq(Register dst, const Operand& src) {
+  EnsureSpace ensure_space(this);
+  emit_rex_64(dst, src);
+  emit(0x0F);
+  emit(0xBD);
+  emit_operand(dst, src);
+}
+
+
 void Assembler::bsfl(Register dst, Register src) {
   EnsureSpace ensure_space(this);
   emit_optional_rex_32(dst, src);