Fix ICE in unnecessary_mut_passed

[lapla-cogito]

fix #12171

changelog: none

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[lapla-cogito]

I suspect (but have no proof) that the cause of this ICE is due to doing something non-trivial. E.g., the following in the original issue:

let mut n = 113383;
while n != 0 {}

I have not been able to add tests because if I include such code as the test, the test will not finish (while I've seen the previous implementation causes ICE🧊), but I'm wondering if it is possible to add test cases that Clippy can validate.

PS, I checked with the following:

fn main() {
    let s = [(); {
        let mut n = 113383;
        while n > 0 {}
        n
    }];
    s.len();
}