Unreachability should detect redundant `isinstance`s even for `object` arguments

[A5rocks]

Bug Report

See #20665

To Reproduce

def f(x: object) -> None:
    if isinstance(x, int):
        print("aaaa")
    elif isinstance(x, int):
        print("aaaa")

Expected Behavior

Error

Actual Behavior

No error

Your Environment

• Mypy version used: master
• Mypy command-line flags: --warn-unreachable --strict
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.13

[A5rocks]

Ugh, nevermind. Properly solving this requires negation types.