RFR: 8229127: Make some methods in the allocation path non-virtual

[Per Liden]

Some virtual methods in the allocation path are no longer overridden by 
any GC, so we can make them non-virtual until such a need arise again. 
Keeping CollectedHeap::array_allocate() virtual because ZGC still wants 
to override that path (as part of JDK-8227226, which is currently out 
for review).

Bug: https://bugs.openjdk.java.net/browse/JDK-8229127
Webrev: http://cr.openjdk.java.net/~pliden/8229127/webrev.0

/Per