checked C++ bindings: add is_ok/is_error methods to isl::checked::stat
checked C++ bindings: add is_ok/is_error methods to isl::checked::stat

This makes isl::checked::stat behave more like isl::checked::boolean,
which has is_error/is_false/is_true methods to check its state.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

PollyLabs/isl,Meinersbur/isl,Meinersbur/isl,PollyLabs/isl,Meinersbur/isl,PollyLabs/isl,PollyLabs/isl,Meinersbur/isl,PollyLabs/isl