[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