[master] RFR: Trim identity-hashcode to 25bit [v2]

[Roman Kennke]

> I'd like to trim the identity hashcode to 25 bits to make room in the upper 32bits for the compressed class-pointer.
> 
> This could in theory regress performance on workloads that make heavy use of i-hashes, but I could not show this in practice (by specjvm workloads like compiler which uses lots of ihashes).
> 
> Next step would be to move the Klass* into the upper 32bit part. (If you look closely, this would be the exact header layout of 32bit JVMs.) And then get rid of the dedicated Klass* field. :-D Eventually I'd make ihash-bits and klass* bits configurable to trade one for the other.
> 
> Testing:
>  - [x] some manual benchmarks
>  - [x] tier1
>  - [ ] tier2

Roman Kennke has updated the pull request incrementally with two additional commits since the last revision:

 - Fix docs about new header layout
 - Adjust testcase to reflect new header layout

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

Changes:
  - all: https://git.openjdk.java.net/lilliput/pull/3/files
  - new: https://git.openjdk.java.net/lilliput/pull/3/files/c3790d0c..c658bb8c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=lilliput&pr=3&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=lilliput&pr=3&range=00-01

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/lilliput/pull/3.diff
  Fetch: git fetch https://git.openjdk.java.net/lilliput pull/3/head:pull/3

PR: https://git.openjdk.java.net/lilliput/pull/3