[mlir][docs] A friendlier improvement for the Toy tutorial chapter 4.

[LyricZ_THU]

Summary

Add notes for discarding private-visible functions in the Toy tutorial chapter 4.

[LyricZ_THU]

Hi, I'm a first-time contributor without the commit accesses. This commit was inspired while I'm following the tutorial to make the Toy language. After I finished the chapter 4, I found my compiler could not discard the function body of func @multiply_transpose, so that the shape inference pass would fail in this function. I'm a beginner for LLVM, it really took me a long time to read the MLIR code and find the visibility problem of my Toy compiler. So I'm wondering whether we could put this note into the docs for friendliness. Thanks!

[LyricZ_THU]

@mehdi_amini Thanks for your reviews! I'm sorry that I am a first-time contributor without a commit access. Could you help me with committing the changes? Thanks :)

mlir/docs/Tutorials/Toy/Ch-4.md

void handleTerminator(Operation *op,
     // Replace the values directly with the return operands.
     assert(returnOp.getNumOperands() == valuesToRepl.size());
     for (const auto &it : llvm::enumerate(returnOp.getOperands()))
       valuesToRepl[it.index()].replaceAllUsesWith(it.value());
   }
 };
 ```
 
+Besides, the inliner will only discard private-visible unused function
+definitions. We also have to set the visibility of functions (except the
+main function) in the MLIR generator.
+
+```c++
+/// Emit a new function and add it to the MLIR module.
+mlir::FuncOp mlirGen(FunctionAST &funcAST) {
+  ...
+  // If this function isn't main, then set the visibility to private.
+  if (funcAST.getProto()->getName() != "main")
+    function.setPrivate();
+
+  return function;
+}
+```
+
 We then register our dialect interface directly on the Toy dialect, similarly to
 how we did for operations.
 
 ```c++
 void ToyDialect::initialize() {
   addInterfaces<ToyInlinerInterface>();
 }
 ```