[lworld] Integrated: 8267428: Several features lack the EnableValhalla guard

[Frederic Parain]

Add if(EnableValhalla) guards around:
  - injection of the IdentityObject interface
  - iteration over fields to pre-load primitive class fields
  - iteration of method arguments to eagerly load primitive class arguments
  - support for flattened and null-free fields in the interpreter

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Commit messages:
 - Add missing if(EnableValhalla) guards

Changes: https://git.openjdk.java.net/valhalla/pull/417/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=417&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267428
  Stats: 169 lines in 3 files changed: 66 ins; 52 del; 51 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/417.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/417/head:pull/417

PR: https://git.openjdk.java.net/valhalla/pull/417