[jdk21u-dev] RFR: 8309697: [TESTBUG] Remove "@requires vm.flagless" from jtreg vectorization tests

[Goetz Lindenmaier]

I backport this for parity with 21.0.3-oracle.

I had to resolve VectorizationTestRunner.java because "8311691: C2: Remove legacy code related to PostLoopMultiversioning" is not in 21.

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Commit messages:
 - Backport a03954e6c57369446ef77136966662780e4b1c4e

Changes: https://git.openjdk.org/jdk21u-dev/pull/226/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=226&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309697
  Stats: 78 lines in 23 files changed: 31 ins; 17 del; 30 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/226.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/226/head:pull/226

PR: https://git.openjdk.org/jdk21u-dev/pull/226