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

[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

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

Commit messages:
 - 8376813: [lworld] Add print for dummy field reused as null-marker

Changes: https://git.openjdk.org/valhalla/pull/2001/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=2001&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8376813
  Stats: 21 lines in 2 files changed: 10 ins; 0 del; 11 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