Fix CopyPropagation regression (2)

[sinkuu]

Remaining part of MIR copyprop regression by (I think) #45380, which I missed in #45753.

fn foo(mut x: i32) -> i32 {
    let y = x;
    x = 123; // `x` is assigned only once in MIR, but cannot be propagated to `y`
    y
}

So any assignment to an argument cannot be propagated.

[rust-highfive]

r? @nikomatsakis

(rust_highfive has picked a reviewer for you, use r? to override)

[arielb1]

@bors r+ rollup

[bors]

📌 Commit 17d6631 has been approved by arielb1