RFR: 8252294: Remove OopsInGenClosure usage from younger_refs_iterate

[Stefan Karlsson]

Hi all,

Please review this patch to remove the usage of OopsInGenClosure from 
younger_refs_iterate and related functions.

https://cr.openjdk.java.net/~stefank/8252294/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8252294

Before the patch we had this call sequence:
  GenCollectedHeap::young_process_roots
  CardTableRS::younger_refs_iterate
  Generation::younger_refs_iterate (CardGeneration as the dynamic type)
  CardGeneration::younger_refs_in_space_iterate
  CardTableRS::younger_refs_in_space_iterate
  ...

and we passed the OopsInGenClosure down the entire way.

After CMS, when we only have one concrete implementation of 
GenCollectedHeap, we can simplify this. I turned the code inside out and 
we now have:
  SerialHeap::young_process_roots (I moved to SerialHeap)
  CardGeneration::younger_refs_iterate
  CardTableRS::younger_refs_in_space_iterate

- Because of this we don't need the virtual 
Generation::younger_refs_iterate, nor the unimplemented version in 
DefNewGeneration. It suffices to have a concrete 
CardGeneration::younger_refs_iterate.

- Inside CardTableRS::younger_refs_in_space_iterate we only used 
OopsInGenClosure to extract the gen_boundary() value. (The value set in 
OopsInGenClosure::set_generation()). Now that I've straightened out the 
code we can now directly extract the "gen boundary" value out of 
CardGeneration (old_gen) instead of passing this information down 
through the closure.

- I moved young_process_roots to SerialHeap, since I need to have access 
to CardGeneration and GenCollectedHeap only provides a Generation* field.

- Note: SerialHeap::young_process_roots still use OopsInGenClosure. I 
have a follow-up patch that changes that. I'd like to keep this patch 
relatively easy.

- Note: There's a lot of dead code and potential cleanups that can be 
made to CardTableRS and GenCollectedHeap. I don't intend to fix that 
with this RFE.

Local testing and currently running through tier1-7 on Linux.

Thanks,
StefanK