[red-knot] Inference for comparison of union types (#13781)
## Summary
Add type inference for comparisons involving union types. For example:
```py
one_or_two = 1 if flag else 2
reveal_type(one_or_two <= 2) # revealed: Literal[True]
reveal_type(one_or_two <= 1) # revealed: bool
reveal_type(one_or_two <= 0) # revealed: Literal[False]
```
closes #13779
## Test Plan
See `resources/mdtest/comparison/unions.md`
