[lworld] RFR: 8354852: [lworld] invalid interval when copying elements in arraycopy

[Frederic Parain]

Fixing computation of the interval of values to be copied when array copying is applied to flat arrays.

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Commit messages:
 - Fix copying interval and test for arraycopy on flat arrays

Changes: https://git.openjdk.org/valhalla/pull/1433/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1433&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8354852
  Stats: 7 lines in 2 files changed: 3 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/valhalla/pull/1433.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1433/head:pull/1433

PR: https://git.openjdk.org/valhalla/pull/1433