RFR: 8364580: Test compiler/vectorization/TestSubwordTruncation.java fails on platforms without RoundF/RoundD

[Jasmine Karthikeyan]

Hi all,
This is a quick patch to fix the test bug where TestSubwordTruncation fails on platforms that don't implement RoundF and RoundD. Thanks!

-------------

Commit messages:
 - Add platform restriction for round IR checks

Changes: https://git.openjdk.org/jdk/pull/26611/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26611&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364580
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/26611.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26611/head:pull/26611

PR: https://git.openjdk.org/jdk/pull/26611