[simulator][MIPS64]: Remove unnecessary DCHECK

After commit a3baa353
`[simulator] Make Call variadic` this DCHECK always fails
for negative input.

Bug: 
Change-Id: Ib0efa8380586ac21b7b8537978310f8397b37ca5
Reviewed-on: https://chromium-review.googlesource.com/859460Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Cr-Commit-Position: refs/heads/master@{#50473}

src/mips64/simulator-mips64.cc

@@ -6873,7 +6873,6 @@ void Simulator::DecodeTypeImmediate() {
       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));