[16] RFR: 8247179: Mechanism for VM operations to not take part in safepoint coalescing

[Erik Österlund]

Hi,

When we have a chain of safepoint operations, we coalesce them to run in 
a single safepoint operation.
Today that is fine, but in the near future, there will be a need for 
certain safepoint operations (my GC safepoints) to not be coalesced to 
run together with other safepoint operations.
The reason relates to concurrent stack scanning, and there are more 
details about the rationale in the bug comments.

I also cleaned up the nested looping when evaluating VM operations in 
safepoints, to be a single loop. I found it unnecessarily hard to deal 
with the nested loop logic.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8247179

Webrev:
http://cr.openjdk.java.net/~eosterlund/8247179/webrev.00/

Thanks,
/Erik