RFR: 8287289: Gather/Scatter with Index Vector [v3]

[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. 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 in the future, which can span a larger-than-4GiB space.
> 
> Please take a look and leave reviews.
> Thanks a lot.

Quan Anh Mai has updated the pull request incrementally with two additional commits since the last revision:

 - remove memory segment support
 - load store from memory segment

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

Changes:
  - all: https://git.openjdk.org/panama-vector/pull/213/files
  - new: https://git.openjdk.org/panama-vector/pull/213/files/e5497060..58ece41f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-vector&pr=213&range=02
 - incr: https://webrevs.openjdk.org/?repo=panama-vector&pr=213&range=01-02

  Stats: 12235 lines in 72 files changed: 5 ins; 12228 del; 2 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