[lworld] RFR: 8268671: [lworld] Wrong code generated for PrimitiveClass.ref.default

[Srikanth Adayapalam]

Lower X.ref.default using aconst_null

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Commit messages:
 - Add test for JDK-8268671
 - 8268671: Wrong code generated for PrimitiveClass.ref.default

Changes: https://git.openjdk.java.net/valhalla/pull/445/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=445&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268671
  Stats: 40 lines in 2 files changed: 39 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/445.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/445/head:pull/445

PR: https://git.openjdk.java.net/valhalla/pull/445