[FIRRTL] Memoize calls to shouldCreateClass in LowerClasses, NFC.

This is already called in multiple places, and we will soon use it in
another situation. This can be expensive, so it is worth memoizing to
avoid repeatedly traversing the module bodies in the slow case.

lib/Dialect/FIRRTL/Transforms/LowerClasses.cpp

@@ -109,6 +109,9 @@ struct LowerClassesPass : public LowerClassesBase<LowerClassesPass> {
   LogicalResult dialectConversion(
       Operation *op, const PathInfoTable &pathInfoTable,
       const DenseMap<StringAttr, firrtl::ClassType> &classTypeTable);
+
+  // State to memoize repeated calls to shouldCreateClass.
+  DenseMap<FModuleLike, bool> shouldCreateClassMemo;
 };
 
 } // namespace
@@ -437,29 +440,45 @@ std::unique_ptr<mlir::Pass> circt::firrtl::createLowerClassesPass() {
 
 // Predicate to check if a module-like needs a Class to be created.
 bool LowerClassesPass::shouldCreateClass(FModuleLike moduleLike) {
-  if (isa<firrtl::ClassLike>(moduleLike.getOperation()))
+  // Memoize calls using the extra state.
+  auto it = shouldCreateClassMemo.find(moduleLike);
+  if (it != shouldCreateClassMemo.end())
+    return it->second;
+
+  if (isa<firrtl::ClassLike>(moduleLike.getOperation())) {
+    shouldCreateClassMemo.insert({moduleLike, true});
     return true;
+  }
 
   // Always create a class for public modules.
-  if (moduleLike.isPublic())
+  if (moduleLike.isPublic()) {
+    shouldCreateClassMemo.insert({moduleLike, true});
     return true;
+  }
 
   // Create a class for modules with property ports.
   bool hasClassPorts = llvm::any_of(moduleLike.getPorts(), [](PortInfo port) {
     return isa<PropertyType>(port.type);
   });
 
-  if (hasClassPorts)
+  if (hasClassPorts) {
+    shouldCreateClassMemo.insert({moduleLike, true});
     return true;
+  }
 
   // Create a class for modules that instantiate classes or modules with
   // property ports.
   for (auto op :
-       moduleLike.getOperation()->getRegion(0).getOps<FInstanceLike>())
-    for (auto result : op->getResults())
-      if (type_isa<PropertyType>(result.getType()))
+       moduleLike.getOperation()->getRegion(0).getOps<FInstanceLike>()) {
+    for (auto result : op->getResults()) {
+      if (type_isa<PropertyType>(result.getType())) {
+        shouldCreateClassMemo.insert({moduleLike, true});
         return true;
+      }
+    }
+  }
 
+  shouldCreateClassMemo.insert({moduleLike, false});
   return false;
 }