match guards with unreachable success blocks ignore fake borrows

Another variant of this, discovered at #t-compiler > Reporting a number of soundness issues in rustc @ 💬

#![forbid(unsafe_code)]
#[inline(never)]
fn read(b: &Box<Box<u64>>) -> u64 { ***b }
fn main() {
    let mut x: Option<Box<Box<u64>>> = Some(Box::new(Box::new(7)));
    match x {
        Some(_) if { x = None; false } && panic!() => {}
        Some(ref b) => println!("{}", read(b)),
        None => println!("none"),
    }
}

Originally posted by @theemathas in #160599

My reading of the problem here is that we insert fake reads for fake borrows into guards' success blocks (just before creating by-value bindings) instead of their failure blocks (before trying more arms, where we need fake borrows for soundness). The guard's success block is unreachable due to the panic, so the fake borrow on x isn't kept live and can't prevent x from being overwritten.