RFR: 8333647: C2 SuperWord: some additional PopulateIndex tests

[Emanuel Peter]

When I did the deep refactoring in https://github.com/openjdk/jdk/pull/19261, I wanted some more tests for `PopulateIndex`. I push them separately to keep the other RFE smaller.

I filed a follow-up RFE for some cases that do not vectorize: https://bugs.openjdk.org/browse/JDK-8332878

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Commit messages:
 - 8333647

Changes: https://git.openjdk.org/jdk/pull/19558/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19558&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333647
  Stats: 97 lines in 2 files changed: 97 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19558.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19558/head:pull/19558

PR: https://git.openjdk.org/jdk/pull/19558