warn-unreachable behaves differently on else-return than on bare return

Bug Report

mypy --warn-unreachable will error on an unreachable return after an if block but not a semantically equivalent else: return ... after an if block. This shows up in platform checking logic, as an example of "unreachable code" which mypy understands and allows.

Based on what I found in existing warn-unreachable issues, there's no report of this same issue.

To Reproduce

Run mypy --warn-unreachable on the two following samples.

This passes:

def foo():
    if sys.platform != "win32":
        return 1
    else:
        return 0

This fails (on Linux/macOS. Use == "win32" to observe on Windows):

def foo():
    if sys.platform != "win32":
        return 1
    return 0

I also put together a small example dir of this in a github repo, if that makes it easier:
https://github.com/sirosen/repro/tree/2a56d3b/mypy-issues

Expected Behavior

mypy should pass on both of these, as they are the same.

Your Environment

• Mypy version used: 0.910

Start by running the two examples with mypy --warn-unreachable and compare how the type checker handles the bare return and the else branch. Trace the unreachable-code analysis and add regression coverage for platform checks; done means both semantically equivalent samples receive the expected result.