RFR: 8076265: Simplify deal_with_reference

[Kim Barrett]

Please review this simplification and cleanup of CMTask::deal_with_reference.

I will need a sponsor for this change.

Removed _CHECK_BOTH_FINGERS_ macro, changing the code to assume it was
true (as it always has been since it was introduced), eliminating some
duplicated code.

Simplfied the current region tests, making use of all the associated
variables either being set or NULL together, so no need for multiple
NULL checks.

Added a helper function for logging and pushing objects on the mark
stack, removing some (almost) code duplication.  This is also the
place where some forthcoming changes for 8069367 will end up.

CR:
https://bugs.openjdk.java.net/browse/JDK-8076265

Webrev:
http://cr.openjdk.java.net/~kbarrett/8076265/webrev/

Testing:
JPRT
local RefWorkload with -XX:+UseG1GC
local JTREG with -XX:+UseG1GC