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.

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

Commit messages:
 - 8375434: [lworld] Cleanup null-marker check for 
InlineKlass::read_payload_from_addr

Changes: https://git.openjdk.org/valhalla/pull/1914/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1914&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375434
  Stats: 7 lines in 1 file changed: 0 ins; 5 del; 2 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

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