OperationLegalizer::isIllegal returns false if operation legality wasn't
registered by user and we expect same behaviour when dynamic legality
callback return None, but instead true was returned.
Differential Revision: https://reviews.llvm.org/D113267
* Change callback signature `bool(Operation *)` -> `Optional<bool>(Operation *)`
* addDynamicallyLegalOp add callback to the chain
* If callback returned empty `Optional` next callback in chain will be called
Differential Revision: https://reviews.llvm.org/D110487