Integrated: 8314181: [lw5] the check for illegal circularity should only be done if value classes are available

[Vicente Romero]

fixing minor missing check from the fix for JDK-8314165

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Commit messages:
 - 8314181: [lw5] the check for illegal circularity should only be done if value classes are available

Changes: https://git.openjdk.org/valhalla/pull/904/files
 Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=904&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314181
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/valhalla/pull/904.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/904/head:pull/904

PR: https://git.openjdk.org/valhalla/pull/904