RFR (S): 8230794: Improve assert to get more information about the JDK-8227695 failure

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that improves the assert that is
failing in JDK-8227695?

We have been unsuccessfully trying to reproduce the problem locally and
doing tens of thousands runs with the original code to get a better
idea of the cause. (Or finding the issue with code inspection or by
tweaking the code)

This change augments the assert to show more information, and adds it
in several places to get a better idea of the cause.

It looks like this issue is caused by extremely specific environmental
conditions we were unable to reproduce.

We will remove this debug code later.

CR:
https://bugs.openjdk.java.net/browse/JDK-8230794
Webrev:
http://cr.openjdk.java.net/~tschatzl/8230794/webrev/
Testing:
hs-tier1-5 without failures :(

Thanks,
  Thomas