Platform check is not remembered after `return`

Bug Report

A platform check that is working fine in if and else is not working after if ... return.

To Reproduce

import sys

def works() -> None:
    if sys.platform != "win32":
        return
    else:
        from winreg import REG_SZ

def fails() -> None:
    if sys.platform != "win32":
        return
    from winreg import REG_SZ

https://mypy-play.net/?mypy=latest&python=3.11&gist=eddad091a6082bd85c5c7524839726b1

Expected Behavior

No error.

Actual Behavior

main.py:12: error: Module "winreg" has no attribute "REG_SZ" [attr-defined]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Failure is on Linux
• Mypy version used: 1.1.1
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.11.2

Start with the provided Python reproducer and mypy-play link, then trace how platform checks and early returns affect type narrowing. Compare the if/else and if ... return cases; done means the second form no longer reports a false winreg.REG_SZ attribute error.