Lint variable only ever assigned a single value

In both the rust source and the rustfmt source I encountered code of the sort

let first = false;
for x in y {
    if first {
        first = false;
        // do some logic
    }
    // do some logic
}

If a variable never gets a mutable reference taken to and all assignments assign the same value, we should have a correctness lint tell the user they are doing something that makes no sense in any program.

This is probably easiest implemented as an MIR lint

Review the Rust source and rustfmt source examples described in the issue, then investigate the MIR lint entry points. Define how to detect variables whose assignments always use the same value and how the correctness lint should report them; done means the behavior is covered by appropriate lint tests.