[Polly] Use "is" instead of "==" to check for None (#94021)

From PEP8
(https://peps.python.org/pep-0008/#programming-recommendations):

> Comparisons to singletons like None should always be done with is or
is not, never the equality operators.

GitOrigin-RevId: b605dab7a8352158ee0d399b8c3433f9a8b495a3
5 files changed