Don't show loop header after break statement

Nobody has claimed this yet.

After a loop reaches a break statement, godebug unnecessarily prints the loop statement again.

```
$ cat min.go
package main

func main() {
_ = "breakpoint"
for i := 1; i < 5; i++ {
break
}
}
$ godebug run min.go
-> _ = "breakpoint"
(godebug) n
-> for i := 1; i < 5; i++ {
(godebug) n
-> break
(godebug) n
-> for i := 1; i < 5; i++ {
(godebug) n
$
```

Though note this doesn't happen if no variables are declared in the loop statement:

```
$ cat min.go
package main

func main() {
_ = "breakpoint"
for {
break
}
}
$ godebug run min.go
-> _ = "breakpoint"
(godebug) n
-> for {
(godebug) n
-> break
(godebug) n
$
```

Reproduce the behavior with the two min.go examples using `godebug run min.go`, then trace the debugger's loop-stepping path. Done means a loop with a declared variable does not print its header again after `break`, while the existing behavior for `for {}` remains correct.

Written by the indexing model from the issue text.