Get rid of the "flipping behaviour" in narrowign by equality/identity

We have this code that makes sure that if the intersection of declared and narrowed is not representable, then we prefer the narrowed. I added this logic a while ago, since this is what a user usually expects in case of e.g. an assignment or even an isinstance() check.

However, the recent effort to make equality/identity narrowing symmetric also caused this to result in a "flipping" behavior, when sides exchange types after a comparison, see e.g. testNarrowGenericCallableEquality. Although such cases are rare, I think the resulting behavior is really weird/confusing.

So I propose to add a flag to each restriction in binder (similar to from_assignment), say prefer_declared that would be set to True for right-most operand in equality/identity. This flag would override the fallback mentioned above. So that after something like assert x == y == a, the type of a will not change if the intersection is not representable.

I understand this breaks the symmetry, but this is purely practical thing, this is why people write x == 2 and not 2 == x.

cc @hauntsaninja

Start with the fallback logic in mypy/checkexpr.py around lines 6494-6499, then trace how restrictions are represented and applied in the binder. Run the testNarrowGenericCallableEquality case to reproduce the flipping behavior and inspect nearby equality/identity narrowing tests. Done means the right-most operand keeps its declared type when the intersection is not representable, without changing ordinary narrowing behavior.