useless_if_let_seq should not warn for composite ifs

[Manishearth]

It warns on:

    let mut bubble_inline_sizes_separately = debug_options.bubble_widths;
    if debug_options.trace_layout {
        paint_threads = 1;
        layout_threads = 1;
        bubble_inline_sizes_separately = true;
    }

While this could be rewritten, I'm not sure if it should.

Additionally, we shouldn't use span_suggestion here since the suggestion isn't valid rust code.

[GnomedDev]

useless_if_let_seq does not exist anymore, and the closest name useless_let_if_seq doesn't trigger for

#![allow(unused_assignments, unused_variables)]
#![warn(clippy::useless_if_let_seq)]

pub fn example(bubble_widths: bool, trace_layout: bool) {
    let paint_threads;
    let layout_threads;

    let mut bubble_inline_sizes_separately = bubble_widths;
    if trace_layout {
        paint_threads = 1;
        layout_threads = 1;
        bubble_inline_sizes_separately = true;
    }
}

Should this be closed?