[lworld] RFR: 8350961: [lworld] Some VarHandles tests fail when field flattening is not applied

[Chen Liang]

#1340 removed null masking for references incorrectly before strict field checks are in place. As a result, we have to mask nulls to default values before the default value concept is gone.

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Commit messages:
 - 8350961: [lworld] Some VarHandles tests fail when field flattening is not applied

Changes: https://git.openjdk.org/valhalla/pull/1385/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1385&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350961
  Stats: 40 lines in 1 file changed: 40 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/valhalla/pull/1385.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1385/head:pull/1385

PR: https://git.openjdk.org/valhalla/pull/1385