[ignition] Fix more -Wsign-compare warnings.

test/unittests/interpreter/bytecode-peephole-optimizer-unittest.cc

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/factory.h"
 #include "src/interpreter/bytecode-label.h"
 #include "src/interpreter/bytecode-peephole-optimizer.h"
 #include "src/objects-inl.h"
@@ skipping 378 matching lines @@
     BytecodeNode first(Bytecode::kLdaZero);
     uint32_t reg_operand = Register(0).ToOperand();
     uint32_t idx_operand = 1;
     BytecodeNode second(operator_replacement[0], reg_operand, idx_operand);
     optimizer()->Write(&first);
     optimizer()->Write(&second);
     Flush();
     CHECK_EQ(write_count(), 1);
     CHECK_EQ(last_written().bytecode(), operator_replacement[1]);
     CHECK_EQ(last_written().operand_count(), 3);
-    CHECK_EQ(last_written().operand(0), 0);
+    CHECK_EQ(last_written().operand(0), 0u);
     CHECK_EQ(last_written().operand(1), reg_operand);
     CHECK_EQ(last_written().operand(2), idx_operand);
     Reset();
   }
 }
 
 }  // namespace interpreter
 }  // namespace internal
 }  // namespace v8