[jdk21u-dev] RFR: 8328938: C2 SuperWord: disable vectorization for large stride and scale

[Aleksey Shipilev]

Unclean backport to prevent accidents in C2 loop optimizations. The patch is unclean, because JDK 21u misses major SuperWord refactorings. I applied the hunk by hand in the similar place, and also used `slp->iv_stride()` in one place to get this thing to work.

@eme64, if you want to take a look at this?

Additional testing:
 - [x] New regression test fails without the patch, passes with it
 - [ ] Linux x86_64 server fastdebug, `all`
 - [ ] Linux x86_64 server fastdebug, 100K Fuzzer tests
 - [ ] Linux x86_64 server fastdebug, Maven CTW

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Commit messages:
 - Backport 2931458711244e20eb7845a1aefcf6ed4206bce1

Changes: https://git.openjdk.org/jdk21u-dev/pull/495/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=495&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328938
  Stats: 272 lines in 2 files changed: 272 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/495.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/495/head:pull/495

PR: https://git.openjdk.org/jdk21u-dev/pull/495