useless_let_if_seq should not trigger on multiple assignments

[sgrif]

The following code triggers this lint:

let thing1;
let thing2;

if things.is_empty() {
    thing1 = None;
    thing2 = None;
} else {
    thing1 = Some(stuff1(things));
    thing2 = Some(stuff2(things));
}

while this can technically be written as a single expression such as

let (thing1, thing2) = if things.is_empty() {
    (None, None)
} else {
    (
        Some(stuff1(things)),
        Some(stuff2(things)),
    )
}

most folks I've polled agree that it's way harder to read, since the binding is so far removed from the expression its receiving. This lint makes complete sense if there's only a single assignment, but I think it should ignore multiple assignments in the same if statement

[jfirebaugh]

This is a duplicate of #2749.