Fix incorrect join in the presence of Any fallback

[hauntsaninja]

Fixes #11925

This avoids mypy from guessing subtype relationships because of the any fallback over here:

mypy/mypy/subtypes.py

Line 438 in e1bfb75

if left.type.fallback_to_any and not self.proper_subtype:

[hauntsaninja]

cc @ilevkivskyi could you review? The check_context assertion comes from #13303 , as far as I can see no changes are required for ignore_type_params=True, but I might be missing something

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

kornia (https://github.com/kornia/kornia)
- kornia/nerf/nerf_model.py:121: error: "object" not callable  [operator]

[ilevkivskyi]

I finally got some time to think about this. In general, those asserts were added when two visitors were merged to prevent accidentally going through previously completely untested code paths. Also I don't really like ignore_type_params for proper subtypes, but after all I think it is not a big deal, so if this doesn't cause problems, let's just keep this.