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

[Wu Yan]

On Thu, 12 Aug 2021 01:48:30 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

>> This is the same as reinterpretD2X. The upper 96 bits of "src" are zero,  I guess it is because when the 128-bit register is initialized, the upper 96 bits will be zeroed, and only its lower 32-bit data will be manipulated later.
>
> I think it's better to clear the higher bits for such cases.

OK, I'll clear the higher bits in the next commit.

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