Remove reachable coverage without counters to maintain invariant that
either there is no coverage at all or there is a live coverage counter
left that provides the function source hash.
The motivating example would be a following closure:
```rust
let f = |x: bool| {
debug_assert!(x);
};
```
Which, with span changes from #93967, with disabled debug assertions,
after the final CFG simplifications but before removal of dead blocks,
gives rise to MIR:
```rust
fn main::{closure#0}(_1: &[closure@a.rs:2:13: 2:22], _2: bool) -> () {
debug x => _2;
let mut _0: ();
bb0: {
Coverage::Expression(4294967295) = 1 - 2;
return;
}
...
}
```
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