[jdk21u-dev] RFR: 8355512: Test compiler/vectorization/TestVectorZeroCount.java times out with -XX:TieredStopAtLevel=3

[Satyen Subramaniam]

Backporting JDK-8355512: Test compiler/vectorization/TestVectorZeroCount.java times out with -XX:TieredStopAtLevel=3. Small patch to TestVectorZeroCount test to make it only execute when C2 is enabled. Backporting for parity with oracle. Ran GHA Sanity Checks, local Tier 1 and 2, and adjusted test directly. Patch is clean.

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Commit messages:
 - Backport 37d04a1e365d005afec3651c5e25fdceeceb9313

Changes: https://git.openjdk.org/jdk21u-dev/pull/2009/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=2009&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8355512
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/2009.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/2009/head:pull/2009

PR: https://git.openjdk.org/jdk21u-dev/pull/2009