Compiler: fix typo in can_propagate_conditional (#60902)
The function was checking `rt.thentype` twice instead of checking both
`rt.thentype` and `rt.elsetype`. This caused incorrect type narrowing
when conditional types were propagated through wrapper functions.
Fixes #60883. Completely done by Claude.
Co-authored-by: Keno Fischer <Keno@users.noreply.github.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
