(🐞) Artificially literal bools not considered when performing reachability checks where branch terminates

[KotlinIsland]

There is already some disparity between naturally literal bools(annotated with Literal or narrowed) and artificially literal bools (platform, python-version, always-true, TYPE_CHECKING etc):

# mypy: always-true=foo
from typing import Literal

foo: bool

if not foo:
    print(1)  # no error ✅

bar: Literal[True]

if not bar:
    print(1)  # error: Statement is unreachable ✅

But this is not currently being applied to this situation:

# mypy: always-true=foo
from typing import Literal

foo: bool

def f1() -> None:
    if foo:
        return
    print(1)  # error: Statement is unreachable ❌

bar: Literal[True]

def f2() -> None:
    if bar:
        return
    print(1)  # error: Statement is unreachable ✅

Key:

✅ = correct behavior
❌ = sussy imposter behavior

Careful of #12325
Related: #11364

[KotlinIsland]

@a_maintainer Can you mark this as bug(🐞), I updated the title, but can't change the tags(😥)