Lint idea: Bind if/else returning bool and then using that binding to conditionally do something

Lint idea:

let x = if [let]? _ {
    {true,false}
} else {
    {false,true}
}

// ... maybe some code not using x

if [!]?x {
    do_something();
}

Can be rewritten as

match _ {
    Some_::Thing(..) => do_something();
    _ => {}
}

or

if _ {
    do_something();
}

What to keep in mind when implementing this:

1. Differ between if and if let: Suggest rewriting it as one if in the first case and as a match in the second*.
2. Between the binding and the usage of the binding is no other use of the binding.
3. Get the conditions right. -> Write many test cases with all combinations of conditions and returns.
4. Maybe this lint should only trigger on if/else blocks returning true/false, without doing anything else inside?
5. I couldn't come up with a lint name, so be creative :)

cc #4308

• Could conflict with the single_match_else lint, but this is a pedantic lint, so we can ignore this

No file or test is named. Start by reviewing existing Clippy lint implementations and test conventions, then use the issue's if/else and if-let examples as cases. Done means the lint's name and exact trigger conditions are settled, including intervening uses and boolean-return combinations, with tests covering the listed cases.