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

[ftynse]

Summary

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 *.

[rriddle]

It's really really sad that we would need to add casting to the same type. I suppose this is better than using SFINAE.

[ftynse]

This is indeed sad, but seemed like least worst solution compared to enable_if.

mlir/include/mlir/IR/Operation.h

 namespace llvm {
 /// Provide isa functionality for operation casts.
 template <typename T> struct isa_impl<T, ::mlir::Operation> {
   static inline bool doit(const ::mlir::Operation &op) {
     return T::classof(const_cast<::mlir::Operation *>(&op));
   }
 };
 
+/// Allow isa<Operation *> on operations.
+template <> struct isa_impl<::mlir::Operation *, ::mlir::Operation> {
+  static inline bool doit(const ::mlir::Operation &op) { return true; }
+};
+
 /// Provide specializations for operation casts as the resulting T is value
 /// typed.
 template <typename T> struct cast_retty_impl<T, ::mlir::Operation *> {
   using ret_type = T;
 };
 template <typename T> struct cast_retty_impl<T, ::mlir::Operation> {
   using ret_type = T;
 };