MIPS64: Add a check to addiu simulation according to the specification.

src/mips64/simulator-mips64.cc

Index: src/mips64/simulator-mips64.cc
diff --git a/src/mips64/simulator-mips64.cc b/src/mips64/simulator-mips64.cc
index c64314c74aaea7819004d132e76d190d30e19f38..3157030847cda4215c09adc0c331ec0bdbd8ad67 100644
--- a/src/mips64/simulator-mips64.cc
+++ b/src/mips64/simulator-mips64.cc
@@ -4413,6 +4413,7 @@ void Simulator::DecodeTypeImmediate(Instruction* instr) {
       break;
     // ------------- Arithmetic instructions.
     case ADDIU: {
+      DCHECK(is_int32(rs));
       int32_t alu32_out = static_cast<int32_t>(rs + se_imm16);
       // Sign-extend result of 32bit operation into 64bit register.
       SetResult(rt_reg, static_cast<int64_t>(alu32_out));