[Docs][NewPM] Add note about required passes

[ychen]

Thank you for writing this down.

llvm/docs/WritingAnLLVMNewPMPass.rst

217

Personally, I may reorder the structure a little bit:

(definition) A pass that defines a static isRequired() method that returns true is a required pass.
< ... example ...>
(motivation) - meta passes (pass manager, adaptor pass etc) - user passes: AlwaysInlinerPass etc.
(behavior) override all other mechanisms that may skip a pass, like "optnone" etc.

[aeubanks]

rework documentation

[ychen]

LGTM

llvm/docs/WritingAnLLVMNewPMPass.rst

.. code-block:: llvm
 
   ; CHECK-NEXT: {{^}}bar{{$}}
   define void @bar() {
     ret void
   }
 
   $ ninja -C build check-llvm
   # runs our new test alongside all other llvm lit tests
+
+FAQs
+====
+
+Required passes
+---------------
+
+A pass that defines a static ``isRequired()`` method that returns true is a required pass. For example:
+
+.. code-block:: c++
+
+  class HelloWorldPass : public PassInfoMixin<HelloWorldPass> {
+  public:
+    PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
+
+    static bool isRequired() { return true; }
+  };
+
+A required pass is a pass that may not be skipped. An example of a required
+pass is ``AlwaysInlinerPass``, which must always be run to preserve
+``alwaysinline`` semantics. Pass managers are required since they may contain
+other required passes.
+
+An example of how a pass can be skipped is the ``optnone`` function
+attribute, which specifies that optimizations should not be run on the
+function. Required passes will still be run on ``optnone`` functions.
+
+For more implementation details, see
+``PassInstrumentation::runBeforePass()``.