Bug report

The code sample should make it obvious, as well as the inconsistency with is_null().
strict-rules is enabled (it was the first thing I thought of).

Code snippet that reproduces the problem

https://phpstan.org/r/55a538eb-af0c-4d10-817d-a19cc255f74b

Expected output

Additional errors should be reported on lines 11 and 14 to report always-true and always-false comparisons respectively.