message: Expound upon Fence's empty implementation being correct.
diff: @@ -4458,7 +4458,13 @@ impl FunctionCodeGenerator<CodegenError> for LLVMFunctionCodeGenerator {
  }
  */
  Operator::Fence { flags: _ } => {
  - // fence is a no-op
  + // Fence is a nop.
  + //
  + // Fence was added to preserve information about fences from
  + // source languages. If in the future Wasm extends the memory
  + // model, and if we hadn't recorded what fences used to be there,
  + // it would lead to data races that weren't present in the
  + // original source language.
  }
  Operator::I32AtomicLoad { ref memarg } => {
  let effective_address = resolve_memory_ptr(
  