Stack overflow when compiling the same code in too many runs

Reproduce with:

``` shell
sbt "run-main dotty.tools.dotc.Bench #runs 10 tests/pos/HelloWorld.scala"
```

Here's the full stack trace (only available if the JVM is started with `-XX:MaxJavaStackTraceDepth=-1` otherwise it's truncated): https://gist.github.com/anonymous/712e94d35fb69faf62fe
