RFR: 8329273: C2 SuperWord: Some basic MemorySegment IR tests [v4]

[Emanuel Peter]

> I could not find any IR vectorization tests for `MemorySegment` yet.
> 
> I make sure to exercise different backing types:
> - arrays
> - buffers
> - native memory
> 
> I filed a follow-up RFE, to eventually make all cases where I have "FAILS" vectorize:
> 
> [JDK-8331659](https://bugs.openjdk.org/browse/JDK-8331659): C2 SuperWord: investicate failed vectorization in compiler/loopopts/superword/TestMemorySegment.java

Emanuel Peter has updated the pull request incrementally with two additional commits since the last revision:

 - Merge branch 'JDK-8329273-memory-segment-ir-tests' of https://github.com/eme64/jdk into JDK-8329273-memory-segment-ir-tests
 - review suggestions by Christian

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18535/files
  - new: https://git.openjdk.org/jdk/pull/18535/files/3cbb4664..b6ddb4b7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18535&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18535&range=02-03

  Stats: 101 lines in 1 file changed: 0 ins; 50 del; 51 mod
  Patch: https://git.openjdk.org/jdk/pull/18535.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18535/head:pull/18535

PR: https://git.openjdk.org/jdk/pull/18535