RFR: 8372641: [s390x] Test failure TestMergeStores.java

[Harshit470250]

[JDK-8347405](https://bugs.openjdk.org/browse/JDK-8347405) introduced a mergeStores optimisation which requires ReverseBytesS opcode and as it was not implemented for s390 the test case is failing.
I also implemented ReverseBytesUS.

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Commit messages:
 - add Opcode ReverseBytesUS
 - Add Opcode ReverseBytesS

Changes: https://git.openjdk.org/jdk/pull/28523/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28523&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372641
  Stats: 34 lines in 1 file changed: 34 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28523.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28523/head:pull/28523

PR: https://git.openjdk.org/jdk/pull/28523