RFR(M): 8077413: Avoid use of Universe::heap() inside collectors

[Per Liden]

Hi,

This patch cleans up the usage of Universe::heap() in our collectors. A 
typical pattern we have is code like this:

   ParallelScavengeHeap* heap = (ParallelScavengeHeap*)Universe::heap();
   assert(heap->kind() == CollectedHeap::ParallelScavengeHeap, "Sanity");

This patch changes this to:

   ParallelScavengeHeap* heap = ParallelScavengeHeap::heap();

There are also other places where Universe::heap() is used, even when 
it's clear what we are inside, e.g. ParallelScavenge-related code. We 
should be using ParallelScavengeHeap::heap() here instead. 
Universe::heap() should (for clarity) only be used by code that doesn't 
know/care which collector we are currently using.

Bug: https://bugs.openjdk.java.net/browse/JDK-8077413

Complete webrev:
http://cr.openjdk.java.net/~pliden/8077413/webrev.0/

If someone finds it easier to review in smaller chunks, I've split it in 
three parts.

Changes made to G1:
http://cr.openjdk.java.net/~pliden/8077413/webrev.0-g1_cleanup/

Changes made to GenCollectedHeap-type collectors:
http://cr.openjdk.java.net/~pliden/8077413/webrev.0-gencollected_heap_cleanup/

Changes made to ParallelScavenge:
http://cr.openjdk.java.net/~pliden/8077413/webrev.0-parallelscavenge_cleanup/


I have two more cleanups in this area, which will be sent out 
separately. These are:
  JDK-8077415 : Remove duplicate variables holding the CollectedHeap
  JDK-8077417 : Cleanup of Universe::initialize_heap()

cheers,
/Per