[turbofan] Implement the call protocol properly for direct calls.

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 63b8daaf1d3fb76af754c610eff68db0d5de5f92..013f77d288ec820b6d9c43bd82158bab5e324f58 100644
--- a/src/mips/macro-assembler-mips.h
+++ b/src/mips/macro-assembler-mips.h
@@ -22,6 +22,7 @@ const Register kInterpreterRegisterFileRegister = {Register::kCode_t3};
 const Register kInterpreterBytecodeOffsetRegister = {Register::kCode_t4};
 const Register kInterpreterBytecodeArrayRegister = {Register::kCode_t5};
 const Register kInterpreterDispatchTableRegister = {Register::kCode_t6};
+const Register kJavaScriptCallArgCountRegister = {Register::kCode_a0};
 const Register kRuntimeCallFunctionRegister = {Register::kCode_a1};
 const Register kRuntimeCallArgCountRegister = {Register::kCode_a0};