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

src/mips/macro-assembler-mips.h

Index: src/mips/macro-assembler-mips.h
diff --git a/src/mips/macro-assembler-mips.h b/src/mips/macro-assembler-mips.h
index 4f6a3c868b7a52c0363e61b21c971ee7e3a5e764..71825cb557d642cf3bcd73db644436fda5f4fe58 100644
--- a/src/mips/macro-assembler-mips.h
+++ b/src/mips/macro-assembler-mips.h
@@ -15,6 +15,7 @@ namespace internal {
 // Give alias names to registers for calling conventions.
 const Register kReturnRegister0 = {Register::kCode_v0};
 const Register kReturnRegister1 = {Register::kCode_v1};
+const Register kReturnRegister2 = {Register::kCode_a0};
 const Register kJSFunctionRegister = {Register::kCode_a1};
 const Register kContextRegister = {Register::kCpRegister};
 const Register kInterpreterAccumulatorRegister = {Register::kCode_v0};