RFR: 8308855: ARM32: TestBooleanVector crashes after 8300257

[Roland Westrelin]

In JDK-8300257, I removed by mistake some logic that's required to
compute the loop alignment. This change puts it back.

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/14508/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14508&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308855
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14508.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14508/head:pull/14508

PR: https://git.openjdk.org/jdk/pull/14508