NFC: Merge OpPass with OperationPass into just OperationPass.

OperationPass' are defined exactly the same way as they are now:
   class DerivedPass :  public OperationPass<DerivedPass>;

OpPass' are now defined as OperationPass, but with an additional template parameter for the operation type:
   class DerivedPass :  public OperationPass<DerivedPass, FuncOp>;

PiperOrigin-RevId: 269122410

mlir/include/mlir/Pass/Pass.h

@@ -227,7 +227,7 @@ template <typename OpT> class OpPassBase : public Pass {
   }
 };
 
-/// Pass to transform an operation.
+/// Pass to transform an operation of a specific type.
 ///
 /// Operation passes must not:
 ///   - modify any other operations within the parent region, as other threads
@@ -237,10 +237,17 @@ template <typename OpT> class OpPassBase : public Pass {
 ///
 /// Derived function passes are expected to provide the following:
 ///   - A 'void runOnOperation()' method.
-template <typename T>
-struct OperationPass : public detail::PassModel<T, Pass> {};
+template <typename PassT, typename OpT = void>
+class OperationPass : public detail::PassModel<PassT, OpPassBase<OpT>> {
+protected:
+  OperationPass()
+      : detail::PassModel<PassT, OpPassBase<OpT>>(OpT::getOperationName()) {}
 
-/// Pass to transform an operation of a specific type.
+  /// Return the current operation being transformed.
+  OpT getOperation() { return cast<OpT>(Pass::getOperation()); }
+};
+
+/// Pass to transform an operation.
 ///
 /// Operation passes must not:
 ///   - modify any other operations within the parent region, as other threads
@@ -250,27 +257,14 @@ struct OperationPass : public detail::PassModel<T, Pass> {};
 ///
 /// Derived function passes are expected to provide the following:
 ///   - A 'void runOnOperation()' method.
-template <typename OpT, typename PassT>
-class OpPass : public detail::PassModel<PassT, OpPassBase<OpT>> {
-protected:
-  OpPass()
-      : detail::PassModel<PassT, OpPassBase<OpT>>(OpT::getOperationName()) {}
-
-  /// Return the current operation being transformed.
-  OpT getOperation() { return cast<OpT>(Pass::getOperation()); }
-};
+template <typename PassT>
+struct OperationPass<PassT, void> : public detail::PassModel<PassT, Pass> {};
 
 /// A model for providing function pass specific utilities.
 ///
-/// Function passes must not:
-///   - read or modify any other functions within the parent module, as
-///     other threads may be manipulating them concurrently.
-///   - modify any state within the parent module, this includes adding
-///     additional functions.
-///
 /// Derived function passes are expected to provide the following:
 ///   - A 'void runOnFunction()' method.
-template <typename T> struct FunctionPass : public OpPass<FuncOp, T> {
+template <typename T> struct FunctionPass : public OperationPass<T, FuncOp> {
   /// The polymorphic API that runs the pass over the currently held function.
   virtual void runOnFunction() = 0;
 
@@ -288,7 +282,7 @@ template <typename T> struct FunctionPass : public OpPass<FuncOp, T> {
 ///
 /// Derived module passes are expected to provide the following:
 ///   - A 'void runOnModule()' method.
-template <typename T> struct ModulePass : public OpPass<ModuleOp, T> {
+template <typename T> struct ModulePass : public OperationPass<T, ModuleOp> {
   /// The polymorphic API that runs the pass over the currently held module.
   virtual void runOnModule() = 0;
 

mlir/include/mlir/Pass/PassManager.h

@@ -64,9 +64,8 @@ class OpPassManager {
     return nest(OpT::getOperationName());
   }
 
-  /// Add the given pass to this pass manager. The pass must either be an opaque
-  /// `OperationPass`, or an `OpPass` that operates on operations of the same
-  /// type as this pass manager.
+  /// Add the given pass to this pass manager. If this pass has a concrete
+  /// operation type, it must be the same type as this pass manager.
   void addPass(std::unique_ptr<Pass> pass);
 
   /// Returns the number of passes held by this manager.

mlir/lib/Pass/Pass.cpp

@@ -225,9 +225,8 @@ OpPassManager &OpPassManager::nest(StringRef nestedName) {
   return nest(OperationName(nestedName, getContext()));
 }
 
-/// Add the given pass to this pass manager. The pass must either be an opaque
-/// `OperationPass`, or an `OpPass` that operates on operations of the same
-/// type as this pass manager.
+/// Add the given pass to this pass manager. If this pass has a concrete
+/// operation type, it must be the same type as this pass manager.
 void OpPassManager::addPass(std::unique_ptr<Pass> pass) {
   // If this pass runs on a different operation than this pass manager, then
   // implicitly nest a pass manager for this operation.