[lworld] RFR: 8375434: [lworld] Add comment for null-check in InlineKlass::read_payload_from_addr [v2]

[Joel Sikström]

> Hello,
> 
> Right now we check the null-marker for the NULLABLE_ATOMIC_FLAT LayoutKind twice, both before and after allocating a heap instance. The first check returns nullptr early if the null-marker is set, as we don't have to allocate an object on the heap if we are going to return nullptr anyways. The other check is redundant, since if the null-marker is not set, the source object is non-null, which means the destination (res) object is non-null as well after copying, so the code inside it is unreachable.
> 
> Testing:
> * Oracle's tier1-4, hotspot_valhalla, jdk_valhalla
> * I also added a `gurantee` for the removed code and ran through hotspot_valhalla and jdk_valhalla locally.

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

  Add comment

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

Changes:
  - all: https://git.openjdk.org/valhalla/pull/1914/files
  - new: https://git.openjdk.org/valhalla/pull/1914/files/705b1416..c4776d84

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1914&range=01
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1914&range=00-01

  Stats: 12 lines in 1 file changed: 12 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/valhalla/pull/1914.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1914/head:pull/1914

PR: https://git.openjdk.org/valhalla/pull/1914