[NFC][mlir] Add support for llvm style casting for mlir types

Note:
when operating on a Type hierarchy with LeafType inheriting from MiddleType which inherits from mlir::Type.
calling LeafType::classof(MiddleType) will always return false.
because classof call the static getTypeID from its parent instead of the dynamic Type::getTypeID
so classof in this context will check if the TypeID of LeafType is the same as the TypeID of MiddleType which is always false.
It is bypassed in this commit inside CastInfo<To, From>::isPossible by calling classof with an mlir::Type.
but other unsuspecting users of LeafType::classof(MiddleType) would still get an incorrect result.