RFR: 8287289: Gather/Scatter with Index Vector

[Quan Anh Mai]

Hi,

This patch implements gather/scatter operations using an index vector in the form of an `IntVector` or a `LongVector` on arrays and memory segments. According to the rationales in the [original PR](https://github.com/openjdk/panama-vector/pull/201), gathering/scattering using a vector is more flexible and can be used to implement the original interface of using an index array easily. Furthermore, 64-bit indexing is necessary for operating on `MemorySegment`s, which can span a larger-than-4GiB space.

Please take a look and leave reviews.
Thanks a lot.

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Commit messages:
 - Merge branch 'gatherscatter' of github.com:merykitty/jdk into gatherscatter
 - copyright year
 - IR tests
 - add tests
 - hotspot implementations
 - first commit

Changes: https://git.openjdk.org/panama-vector/pull/213/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-vector&pr=213&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8287289
  Stats: 28816 lines in 99 files changed: 21429 ins; 2654 del; 4733 mod
  Patch: https://git.openjdk.org/panama-vector/pull/213.diff
  Fetch: git fetch https://git.openjdk.org/panama-vector.git pull/213/head:pull/213

PR: https://git.openjdk.org/panama-vector/pull/213