[mlir] Fix an unused variable warning

This patch fixes:

  mlir/test/lib/IR/TestBytecodeRoundtrip.cpp:403:10: error: unused
  variable 'status' [-Werror,-Wunused-variable]

Author: kazutakahirata

mlir/test/lib/IR/TestBytecodeRoundtrip.cpp

@@ -402,6 +402,7 @@ struct TestBytecodeRoundtripPass
     // Downgrade IR constructs before writing the IR to bytecode.
     auto status = downgradeToVersion(op, targetEmissionVersion);
     assert(succeeded(status) && "expected the downgrade to succeed");
+    (void)status;
 
     BytecodeWriterConfig writeConfig;
     writeConfig.setDialectVersion<test::TestDialect>(