RFR: 8316594: C2 SuperWord: wrong result with hand unrolled loops

[Emanuel Peter]

Please read description in https://github.com/openjdk/jdk/pull/15864.

Tier1-6 + stress-testing (Running)
Performance testing (Running)

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Commit messages:
 - whitespace
 - rm avx2 flag, not necessary
 - more comments
 - beautify code
 - fix tests
 - 8316594: C2 SuperWord: wrong result with hand unrolled loops

Changes: https://git.openjdk.org/jdk/pull/15866/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15866&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316594
  Stats: 229 lines in 4 files changed: 219 ins; 3 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/15866.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15866/head:pull/15866

PR: https://git.openjdk.org/jdk/pull/15866