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

[Xiaohong Gong]

On Tue, 10 Aug 2021 11:51:20 GMT, Wu Yan <wuyan at openjdk.org> wrote:

>> There are several places in the AArch64 back end where we expect the upper bits of a register to be zero, but we've never depended on it. This is not a good time to start, so let's clear the bits in order to be certain.
>
> 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.

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