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

[Jasmine Karthikeyan]

Hi all,
This is a small patch to TestVectorZeroCount to make it only execute when C2 is enabled, to fix a timeout with -XX:TieredStopAtLevel=3. This test takes a long time to finish without C2 because it iterates through all of the integers twice. Since the intention of the test is to stress the C2-specific `numberOfLeadingZeros` and `numberOfTrailingZeros` intrinsics, I think it makes sense to limit it to running with C2 only.

Reviews would be appreciated!

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Commit messages:
 - Only run TestVectorZeroCount with C2

Changes: https://git.openjdk.org/jdk/pull/25243/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25243&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/jdk/pull/25243.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25243/head:pull/25243

PR: https://git.openjdk.org/jdk/pull/25243