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

The support for this has been added by 946311b8938114a37db5c9d42fb9f5a1481ccae1
but then ignored by bc22b5c9a2f729460ffdf7627b3534a8d9f3f767.

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 *`.

Differential Revision: https://reviews.llvm.org/D125543

Reviewed by: rriddle

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