[Interpreter] Add ForInPrepare runtime function which returns a ObjectTriple.

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
diff --git a/src/x64/macro-assembler-x64.h b/src/x64/macro-assembler-x64.h
index aefc53f8a2fcaa7ac00ca7c9392a4c0b06808f89..fc235a11655b316ef24e9e288a4d19a003b339ca 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -18,6 +18,7 @@ namespace internal {
 // Give alias names to registers for calling conventions.
 const Register kReturnRegister0 = {Register::kCode_rax};
 const Register kReturnRegister1 = {Register::kCode_rdx};
+const Register kReturnRegister2 = {Register::kCode_r8};
 const Register kJSFunctionRegister = {Register::kCode_rdi};
 const Register kContextRegister = {Register::kCode_rsi};
 const Register kInterpreterAccumulatorRegister = {Register::kCode_rax};