block ignore annotation should be invalid when beyond its scope

For following case, the block ignore will fall into block in `func bar`. It should be treated as invalid annotation.

```go
func foo() {
if true {
fmt.Println()
}

fmt.Println()
//+gocover:ignore:block want to ignore here
}

func bar() {
}
```

Use the Go example to reproduce how a //+gocover:ignore:block annotation is applied after its intended scope. Trace the block-ignore handling and verify that the annotation is reported as invalid rather than affecting func bar.

Written by the indexing model from the issue text.