minor: check original type for replace_arith_op

[A4-Tacks]

Fixup #22180

It's strange that the unit test can pass, but it cannot be offer in the editor

fn main() {
    let mut x = 1;
    x $0+= 2;
}

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.