[turbofan] Fix typo in IrOpcode::Mnemonic.

test/unittests/compiler/opcodes-unittest.cc

+// Copyright 2015 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 <cstring>
+
+#include "src/compiler/opcodes.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace v8 {
+namespace internal {
+namespace compiler {
+
+TEST(IrOpcodeTest, Mnemonic) {
+  EXPECT_STREQ("UnknownOpcode",
+               IrOpcode::Mnemonic(static_cast<IrOpcode::Value>(123456789)));
+#define OPCODE(Opcode) \
+  EXPECT_STREQ(#Opcode, IrOpcode::Mnemonic(IrOpcode::k##Opcode));
+  ALL_OP_LIST(OPCODE)
+#undef OPCODE
+}
+
+}  // namespace compiler
+}  // namespace internal
+}  // namespace v8