RFR(S): 8154452: Deferred cleanups after split of G1CollectorPolicy code

[Mikael Gerdin]

Hi all,

Please review this small set of cleanups brought up during the review of 
8154154.

I've decided to simply remove {is,as}_g1_policy since they had no 
callers and the code instead checks the UseG1GC flag to determine if G1 
is running.

As Kim suggested, CollectorPolicy::post_heap_initialize does no useful 
work for neither G1 nor other collectors and I decided to remove it.
Removing post_heap_initialize allowed me to remove its only caller, 
ColelctedHeap::post_initialize and I made that method pure virtual.

I also moved the setting of G1HeapRegionSize closer to where the actual 
value is computed. This allowed me to get rid of 
G1CollectorPolicy::initialize_flags, thus making G1CollectorPolicy 
implement a single pure virtual method, initialize_alignments().

Bug: https://bugs.openjdk.java.net/browse/JDK-8154452
Webrev: http://cr.openjdk.java.net/~mgerdin/8154452/webrev.0
Testing: RBT GC Testing

/Mikael