RFR: 8275527: Refactor forward pointer access [v5]

[Roman Kennke]

> Accessing the forward pointer is currently a little inconsistent. Some code paths call oopDesc::forwardee() / oopDesc::is_forwarded(), some code paths call forwardee() and check it for ==/!= NULL, some code paths even call markWord::decode_pointer() and markWord::is_marked() instead.
> 
> This change attempts to make the situation more consistent. For simple cases it preserves oopDesc::forwardee() / is_forwarded(), some cases need to use the markWord for consistency in concurrent GC, they now use markWord::forwardee() and markWord::is_forwarded(). Also, checking whether or not an object is forwarded is now consistently done using is_forwarded() and not by checking forwardee ==/!= NULL. This also resolves the mess in G1 full GC that changes not-forwarded objects to have a NULL (fake-) pointer. This is not necessary, because we can just as well use the lock bits to determine whether or not the object is forwarded.
> 
> Testing:
>  - [x] tier
>  - [x] tier2
>  - [x] hotspot_gc

Roman Kennke has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 14 commits:

 - Merge branch 'master' into optimize-fwdptr
 - Don't use forwarded terminology in markWord
 - Move forward impl into markWord and add assert
 - Fix Parallel GC mistake
 - Revert unnecessary changes
 - Update some copyright headers
 - Add missing includes
 - Merge branch 'master' into optimize-fwdptr
 - Add missing includes
 - Rename mwd -> fwd
 - ... and 4 more: https://git.openjdk.java.net/jdk/compare/a0b84453...d63962a3

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Changes: https://git.openjdk.java.net/jdk/pull/5955/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5955&range=04
  Stats: 46 lines in 9 files changed: 4 ins; 26 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5955.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5955/head:pull/5955

PR: https://git.openjdk.java.net/jdk/pull/5955