RFR: 8221648: Remove CollectedHeap::is_in_closed_subset()

[Per Liden]

I propose that we remove CollectedHeap::is_in_closed_subset() and its 
unused sibling CollectedHeap::is_in_closed_subset_or_null(). We can 
replace all calls to is_in_closed_subset(), most of which is in 
verification code, with calls to is_in(). For Serial and ZGC, 
is_in_closed_subset() is already equivalent to is_in(). For G1, 
is_in_closed_subset() is very close to equivalent to is_in(). For CMS, 
Parallel, Epsilon, and Shenandoah, is_in_closed_subset() is equivalent 
to is_in_reserved(). However, their is_in() is arguably not a lot more 
expensive, at least not to the degree that is motivates keeping the 
is_in_closed_subset() API around. G1 is the only GC that calls 
is_in_closed_subset() outside of verification code (in 
RegisterHumongousWithInCSetFastTestClosure).

Bug: https://bugs.openjdk.java.net/browse/JDK-8221648
Webrev: http://cr.openjdk.java.net/~pliden/8221648/webrev.0

/Per