RFR: 8144076: Move evac failure handling and reference processing to post_evacuate_collection_set

[Erik Helin]

Hi all,

this small patch moves the evacuation failure handling code and the
reference processing code from G1CollectedHeap::evacuate_collection_set
to G1CollectedHeap::post_evacuate_collection_set, in order to keep
evacuate_collection_set only about evacuation.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8144076

Webrev:
http://cr.openjdk.java.net/~ehelin/8144076/webrev.00/

Please note this patch builds upon the patch for 8144075.

Testing:
- GC test suite with full verification.

Thanks,
Erik