cgen: fix fn call with mut reference args (fix #21265)

[yuyi98]

This PR fix fn call with mut reference args (fix #21265).

• Fix fn call with mut reference args.
• Add test.

@[heap]
struct Client {
mut:
    next &Client = unsafe { nil }
    prev &Client = unsafe { nil }
}

fn init_vm1(mut head &Client) {
    for c := head; c; c = c.next {
    }
}

fn init_vm2(mut head &Client) {
    for c := head; c == unsafe { nil }; c = c.next {
    }
}

fn main() {
    mut head := &Client{}
    init_vm1(mut head)
    init_vm2(mut head)
    assert true
}

PS D:\Test\v\tt1> v run .