[lworld] RFR: 8376813: [lworld] Add print for dummy field reused as null-marker [v2]

[Joel Sikström]

> Hello,
> 
> Right now it's not clear that the dummy field that is injected into empty inline klasses can be/is reused for the null-marker when looking at the printed layout from -XX:+PrintInlineLayout. I suggest we enhance this print to indicate if the dummy field has been reused for the null-marker.
> 
> I've tested that the added comment in the print is there when nullability is turned on (default), and not there when turned off (`-XX:-UseNullableValueFlattening -XX:-UseNullableNonAtomicValueFlattening`).
> 
> 
> Before:
> Instance fields:
>  @0 RESERVED 12/-
>  @12 REGULAR 1/1 ".empty" B
> 
> After:
> Instance fields:
>  @0 RESERVED 12/-
>  @12 REGULAR 1/1 ".empty" B (reused as null-marker)
> 
> 
> Testing:
> * GHA
> * Oracle's tier1

Joel Sikström has updated the pull request incrementally with one additional commit since the last revision:

  no dummy field in static layout printing

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

Changes:
  - all: https://git.openjdk.org/valhalla/pull/2001/files
  - new: https://git.openjdk.org/valhalla/pull/2001/files/a94f1bf5..87849f24

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=2001&range=01
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=2001&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/valhalla/pull/2001.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2001/head:pull/2001

PR: https://git.openjdk.org/valhalla/pull/2001