[docs][NewPM] Add example C++ code on how to actually use the new PM

[asbirlea]

Thank you, this is very helpful!

Feedback for future extensions: how to add sanitizers to the default pipeline.

llvm/docs/NewPassManager.rst

44

Sync so both comment and optimization level are either O3 or O2.

llvm/docs/NewPassManager.rst

 ==========================
 Using the New Pass Manager
 ==========================
 
 .. contents::
     :local:
 
 Overview
 ========
 
 For an overview of the new pass manager, see the `blog post
 <https://blog.llvm.org/posts/2021-03-26-the-new-pass-manager/>`_.
 
+Just Tell Me How To Run The Default Optimization Pipeline With The New Pass Manager
+===================================================================================
+
+.. code-block:: c++
+
+  // Create the analysis managers.
+  LoopAnalysisManager LAM;
+  FunctionAnalysisManager FAM;
+  CGSCCAnalysisManager CGAM;
+  ModuleAnalysisManager MAM;
+
+  // Create the new pass manager builder.
+  // Take a look at the PassBuilder constructor parameters for more
+  // customization, e.g. specifying a TargetMachine or various debugging
+  // options.
+  PassBuilder PB;
+
+  // Make sure to use the default alias analysis pipeline, otherwise we'll end
+  // up only using a subset of the available analyses.
+  FAM.registerPass([&] { return PB.buildDefaultAAPipeline(); });
+
+  // Register all the basic analyses with the managers.
+  PB.registerModuleAnalyses(MAM);
+  PB.registerCGSCCAnalyses(CGAM);
+  PB.registerFunctionAnalyses(FAM);
+  PB.registerLoopAnalyses(LAM);
+  PB.crossRegisterProxies(LAM, FAM, CGAM, MAM);
+
+  // Create the pass manager.
+  // This one corresponds to a typical -O2 optimization pipeline.
+  ModulePassManager MPM = PB.buildPerModuleDefaultPipeline(OptimizationLevel::O2);
+
+  // Optimize the IR!
+  MPM.run(MyModule, MAM);
+
+The C API also supports most of this, see ``llvm-c/Transforms/PassBuilder.h``.
+
 Adding Passes to a Pass Manager
 ===============================
 
 For how to write a new PM pass, see :doc:`this page <WritingAnLLVMNewPMPass>`.
 
 To add a pass to a new PM pass manager, the important thing is to match the
 pass type and the pass manager type. For example, a ``FunctionPassManager``
 can only contain function passes: