RFR: 8372641: [s390x] Test failure TestMergeStores.java [v2]

[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.

Harshit470250 has updated the pull request incrementally with one additional commit since the last revision:

  Apply suggestions from code review
  
  Review
  
  Co-authored-by: Manuel Hässig <manuel at haessig.org>
  Co-authored-by: Amit Kumar <amitlocham09 at icloud.com>

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Changes:
  - all: https://git.openjdk.org/jdk/pull/28523/files
  - new: https://git.openjdk.org/jdk/pull/28523/files/36c6de13..61352bdc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28523&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28523&range=00-01

  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 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