[MLIR][NFC] Remove unnecessary cast.

I was reading this post:
https://www.fluentcpp.com/2017/05/19/crtp-helper/

And I noticed that most likely this cast is not needed.
Unless it is needed by some compiler versions.
I tested it with:
cmake --build . --target check-mlir

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D121555

Author: antonio-cortes-perez

mlir/include/mlir/IR/OpDefinition.h

@@ -322,14 +322,8 @@ class TraitBase {
 protected:
   /// Return the ultimate Operation being worked on.
   Operation *getOperation() {
-    // We have to cast up to the trait type, then to the concrete type, then to
-    // the BaseState class in explicit hops because the concrete type will
-    // multiply derive from the (content free) TraitBase class, and we need to
-    // be able to disambiguate the path for the C++ compiler.
-    auto *trait = static_cast<TraitType<ConcreteType> *>(this);
-    auto *concrete = static_cast<ConcreteType *>(trait);
-    auto *base = static_cast<OpState *>(concrete);
-    return base->getOperation();
+    auto *concrete = static_cast<ConcreteType *>(this);
+    return concrete->getOperation();
   }
 };