RFR: 8329273: C2 SuperWord: Some basic MemorySegment IR tests

[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

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Commit messages:
 - fix tabs
 - speed up test
 - small cosmetic fix
 - make things static
 - long loop tests
 - handle AlignVector
 - int cases
 - int-index case
 - disable mixed tests
 - mixed
 - ... and 14 more: https://git.openjdk.org/jdk/compare/ea3909ac...b6f16a58

Changes: https://git.openjdk.org/jdk/pull/18535/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18535&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329273
  Stats: 860 lines in 1 file changed: 860 ins; 0 del; 0 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