[vectorIntrinsics] RFR: RFC: Vector API masking support proposal for Arm SVE [v2]

[Xiaohong Gong]

> Please help to review this proposal for Vector API masking support. 
> 
> This is the masking part of https://bugs.openjdk.java.net/browse/JDK-8261663 - the second incubator for JEP 338 Vector API. As the JEP described, the masked vector operations are currently implemented by composing the non-masked operation with a blend operation. This can be improved by using the hardware mask feature on supported architecture like Arm SVE and X86 AVX-512. So here is the proposals for Arm SVE.  We assume the ideas could also be applied to X86 AVX-512.
> 
> To support the masking feature, this PR added the following implementations:
>   -  SVE predicate register allocation
>   -  Mask type and basic mask IR definition
>   -  Mask implementation for masked vector store
>   -  Mask implementation for masked binary operations
> 
> For the masked binary operations, we have created two proposals for discussion:
>   -  By mainly changing the C2 compiler
>   -  By improving the Vector API Java implementation together with simpler C2 compiler changes
>   
> This PR shows the second solution since we think this solution is better.  But we also have a prototype for the first solution. Please see: https://github.com/XiaohongGong/panama-vector/commit/372489feeae06bc53c46709d389cb0e46e9fb4f6 . The basic support changes are shared with this PR.
> 
> This PR doesn't contain all the masking support changes. There are still too many missing parts that we will continue working on, including:
>   -  Mask support for other operations (unary,ternary,reduction,load,etc.)
>   -  More mask IRs implementation (maskAll, toVector, allTrue, anyTrue, trueCount, eq, etc)
>   -  Better solution for vector mask load/store  (the memory type is boolean)
>   -  Vector boxing/unboxing support for mask type (deoptimization?)
>   -  Tail loop elimination?
> 
> It's worth to mention that this PR mainly provides the proposals for SVE masking support, and any suggestions and discussions are welcome! Thanks a lot!

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

 - Add mask support for masked vector store
 - AArch64 backend implementation for masked IRs
 - Add SVE specific load/store vector mask IRs
 - Split one "VectorMaskCmpNode" into two nodes
 - Basic support for mask
 - SVE predicate register allocation support

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Changes: https://git.openjdk.java.net/panama-vector/pull/40/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-vector&pr=40&range=01
  Stats: 2582 lines in 78 files changed: 2232 ins; 117 del; 233 mod
  Patch: https://git.openjdk.java.net/panama-vector/pull/40.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-vector pull/40/head:pull/40

PR: https://git.openjdk.java.net/panama-vector/pull/40