[lworld] RFR: 8249869: [lworld] Remove LW2 specific code from the JIT

[Tobias Hartmann]

The concept of flattenable fields needs some cleanup (inline types are always flattenable/null-free). We can remove the
side-channel introduced by JDK-8212190. See JDK-8247357 for details.

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Commit messages:
 - 8249869: [lworld] Remove LW2 specific code from the JIT

Changes: https://git.openjdk.java.net/valhalla/pull/116/files
 Webrev: https://webrevs.openjdk.java.net/valhalla/116/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8249869
  Stats: 362 lines in 36 files changed: 16 ins; 193 del; 153 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/116.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/116/head:pull/116

PR: https://git.openjdk.java.net/valhalla/pull/116