[lworld] Integrated: 8256929: [lworld] Fails to compile with --with-debug-level=optimized

[David Simms]

Guard use of debug "valid_itable_index()" assert macro

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Commit messages:
 - 8256929: [lworld] Fails to compile with --with-debug-level=optimized

Changes: https://git.openjdk.java.net/valhalla/pull/276/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=276&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256929
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/276.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/276/head:pull/276

PR: https://git.openjdk.java.net/valhalla/pull/276