RFR: 8278568: Consolidate filler objects [v2]

[Roman Kennke]

> In PSPromotionLAB exists some code that fills unused tails of promotion LABs, which is semantically the same as the CollectedHeap::fill* family of methods. They can be consolidated.
> 
> We also have ContiguousSpace::allocate_temporary_filler() which does the same, but is unused and can be removed. 
> 
> Testing:
>  - [x] hotspot_gc
>  - [x] tier1
>  - [ ] tier2
>  - [ ] tier3

Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:

  Make min_dummy_object_size() constexpr, use trueInDebug

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6814/files
  - new: https://git.openjdk.java.net/jdk/pull/6814/files/f763bbaf..ebec77d9

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6814&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6814&range=00-01

  Stats: 9 lines in 4 files changed: 2 ins; 4 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6814.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6814/head:pull/6814

PR: https://git.openjdk.java.net/jdk/pull/6814