RFC: Separate/refactor forward pointer access

[Roman Kennke]

Hello GC-devs,

I implemented a refactoring and separation of the forward-pointer access 
that currently is in oopDesc. My original intent was to have this as an 
optimization (see rationale in the PR below in oopForwarding.hpp), but 
alas, I could not show it to improve any performance.

However, maybe we want to have it anyway, on the basis of cleanliness 
and separation of concerns:

https://github.com/openjdk/jdk/pull/5955

I like that it encapsulates and scopes header access and forwarding 
pointer decoding. I also like that it's separated out of oopDesc and 
markWord. In some places, it replaces otherwise aweful raw decoding of 
forwarding pointer (those should be fixed, no matter what). And I also 
like that it avoids double-loading the header, and compiles better code, 
even if it does not make a dent in performance.

Opinions about it?

If it gets rejected, no problem at all. I actually almost wanted to 
abandon it, but then thought it cannot hurt to ask first.

Also, in any case, I would like to fix the mess that is in 
g1FullGCCompactionPoint.cpp and 
src/hotspot/share/gc/g1/g1FullGCOopClosures.inline.hpp, I stumbled 
across this multiple times already, and couldn't really make heads or 
tails with it. We shouldn't reset the header of non-forwarded objects 
there, just to be able to make == NULL comparison of a raw-decoded 
forward pointer elsewhere. If this PR gets rejected, I'd fix it separately.

If we agree that it's useful, I'd formalize it and open an RFR.

Thanks,
Roman