RFR: 8372136: VectorAPI: Refactor subword gather load API java implementation [v2]

[Xiaohong Gong]

> The current subword (`byte`/`short`) gather load API implementation is not well-suited for platforms that provide native vector instructions for these operations. As **discussed in PR [1]**, we'd like to re-implement these APIs with a **unified cross-platform** solution.
> 
> The main idea is to re-implement the API at Java-level, by performing multiple sub-gather operations. Each sub-gather operation loads a portion of elements using a specific index vector by calling the HotSpot intrinsic API. The partial results are then merged using vector `slice` and `or` operations. This design simplifies the VM compiler intrinsic implementation and better aligns with the Vector API design principles.
> 
> Key changes:
> 1. Re-implement the subword gather load API at the Java level. The HotSpot intrinsic `VectorSupport.loadWithMap` is simplified by reducing the vector index parameters from four (vix1-vix4) to a single parameter.
> 2. Adjust the compiler intrinsic implementation to support the new Java API, including updates to the x86 backend implementation.
> 
> The performance impact varies across different scenarios on X86. I tested the performance with different AVX levels on an X86 machine that supports AVX512. To achieve optimal performance, I also **applied PR [2]**, which improves the performance of the **`slice()`** API on X86. Following is the summarized performance gains, where:
> 
> - "non masked" means the gather operation is not the masked gather API.
> - "masked" means the gather operation is the masked gather API.
> - "1 gather cases" means the gather API is implemented with a single gather operation. E.g. Load `Short128Vector` with `MaxVectorSize=256`.
> - "2 gather cases" means the gather API is implemented with 2 parts of gather operations. E.g. Load `Short256Vector` with `MaxVectorSize=256`.
> - "4 gather cases" means the gather API is implemented with 4 parts of gather operations. E.g. Load `Byte256Vector` with `MaxVectorSize=256`.
> - "Un-intrinsified" means the gather operation is not supported to be intrinsified by hotspot. E.g. Load `Byte512Vector` with `MaxVectorSize=256`. The singificant performance uplifts comes from the Java-level changes which removes the vector index generation and range checks for such cases.
> 
> 
> ----------------------------------------------------------------------------
>                |          UseAVX=3           |          UseAVX=2           |
>                |-----------------------------|-----------------------------|
>                |  non maske...

Xiaohong Gong has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - Merge 'jdk:master' into JDK-8372136
 - 8372136: VectorAPI: Refactor subword gather load API java implementation

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Changes: https://git.openjdk.org/jdk/pull/28520/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28520&range=01
  Stats: 558 lines in 13 files changed: 383 ins; 78 del; 97 mod
  Patch: https://git.openjdk.org/jdk/pull/28520.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28520/head:pull/28520

PR: https://git.openjdk.org/jdk/pull/28520