[mlir] support isa/cast/dyn_cast<Operation *>(operation)

This enables one to write generic code that can be instantiated for both
specific operation classes and the common base class without
specialization. Examples include functions that take/return ops, such
as:

```mlir
template <typename FnTy>
void applyIf(FnTy &&lambda, ...) {
  for (Operation *op : ...) {
    auto specific = dyn_cast<function_traits<FnTy>::template arg_t<0>>(op);
    if (specific)
      lambda(specific);
  }
}
```

that would otherwise need to rely on template specialization to support
lambdas that take specific operations and those that take `Operation *`.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D124675