RFR: 8259948: Aarch64: Add cast nodes for Aarch64 Neon backend [v2]

[Eric Liu]

On Fri, 6 Aug 2021 09:48:50 GMT, Wang Huang <whuang at openjdk.org> wrote:

>> * In this issue, we plan to complete all missing implementation for aarch64 neon backend. For example, cast from Byte to Long, cast from Long to Byte, and so on.
>> * It may be a solver of JDK-8269866, or part of it.
>
> Wang Huang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fix code style

src/hotspot/cpu/aarch64/aarch64_neon.ad line 195:

> 193:     // If registers are the same, no register move is required - the
> 194:     // upper bits of "src" are expected to have been initialized
> 195:     // to zero.

I have a little concern about this assumption. How to ensure the upper bits are zero? Since the `ReinterpretNode` could be used separately and not always with `CastNode` together. E.g

https://github.com/openjdk/jdk/blob/jdk-18%2B9/src/hotspot/share/opto/vectorIntrinsics.cpp#L844

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PR: https://git.openjdk.java.net/jdk/pull/4839