Fix objectmode is/is not

User reported the error https://groups.google.com/a/continuum.io/forum/?utm_medium=email&utm_source=footer#!msg/numba-users/3KwP6FVakBs/NawO3DcJqHMJ

Reason: `is` and `is not` were not handled in object mode.
That assert is really pointless, IMHO.