(🐞) unreachable `isinstance` only works with variables

# mypy: warn-unreachable
def foo() -> int: return 1
class A:
     i: int
     foo() -> int: return 1
assert isinstance(1, str)
assert isinstance(1 + 1, str)
assert isinstance(foo(), str)
assert isinstance(A.foo(), str)
assert isinstance(A().foo(), str)
assert isinstance(A().i, str)
a: A
assert isinstance(a.foo(), str)

i = 1    
assert isinstance(i, str)  # error: Subclass of "int" and "str" cannot exist: would have incompatible method signatures  [unreachable]

Start by running the provided mypy snippet with warn-unreachable enabled and compare the isinstance expressions that do and do not produce diagnostics. Trace the type-checking path for expressions versus variables; done should mean the intended equivalent expressions receive consistent unreachable checks.