RFR: 8166663: Simplify oops_on_card_seq_iterate_careful interface

[Kim Barrett]

Please review this simplification of the interface and implementation
of HeapRegion::oops_on_card_seq_iterate_careful. It only has one
caller, G1RemSet::refine_card, which does not use some of the
presently supported generality.

By making these changes we simplify the analysis and code changes
required to address the main task of JDK-8160369.
 
Specific issues include:

(1) This function has a bool filter_young parameter, which is used to
control and conditionalize its behavior. However, the only caller
always provides a true value for that parameter.

(2) This function returns either NULL (to indicate success) or the
start address of a possibly incompletely initialized object that
caused heap parsing to fail. The caller uses this value to decide
whether the operation was successful (NULL), but doesn't use the
actual failure value (the address where parsing failed). Instead,
there is a comment suggesting a more complex to implement response to
failure might be used in the future.

Unlike CMS, with G1 the situation where parsing may fail is quite
rare. This will be discussed further with the fix for JDK-8166607. So
extra work here isn't worthwhile, and the present response of just
requeuing the card for later reconsideration is adequate.

Since the caller presently only uses the result as a (inverted)
boolean success/failure, changing the result to a (non-inverted)
success/failure indication further simplifies both the caller and the
callee.

CR:
https://bugs.openjdk.java.net/browse/JDK-8166663

Webrev:
http://cr.openjdk.java.net/~kbarrett/8166663/webrev.00/

Testing:
JPRT, local jtreg hotspot_all