```
  if   L1
L0:
  jump L2
L1:
  ...
L2:
```

was wrongly optimized to:

```
  unless L2
L0:
L1:
  ...
L2:
```

To make it conservative, this optimization is now disabled when there is
any label between `if` and `jump` instructions.
Fixes [Bug #14897].

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63868 b2dd03c8-39d4-4d8f-98ff-823fe69b080e