Change `if_same_then_else` to be a `style` lint
CC #3770
From https://github.com/rust-lang/rust-clippy/issues/3770#issuecomment-687565594 (`@flip1995):`
> Oh I thought I replied to this: I definitely see now that having this
> as a correctness lint might be the wrong categorization. What we might
> want to do is to just allow this lint, if there are comments in the
> arm bodies. But a good first step would be to downgrade this lint to
> style or complexity. I would vote for style since merging two arms is
> not always less complex.
changelog: [`if_same_then_else`]: Change to be a `style` lint
3ea5bcf5ee