RFR: 8256318: AArch64: Add support for floating-point absolute difference [v4]

[Dong Bo]

On Tue, 17 Nov 2020 03:47:37 GMT, Ningsheng Jian <njian at openjdk.org> wrote:

>> Dong Bo has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   move ABS/FABS/FABD neon vector rules into aarch64_neon.ad
>
> src/hotspot/cpu/aarch64/aarch64_neon.ad line 3462:
> 
>> 3460: instruct vabs8B(vecD dst, vecD src)
>> 3461: %{
>> 3462:   predicate(n->as_Vector()->length() == 8);
> 
> This is different from original code.

For integer absolute (vector), the accepted arrangements are `T8B, T16B, T4H, T8H, T2S, T4S, T2D`.
ARM compiler armasm user guide reference: https://developer.arm.com/documentation/dui0801/h/A64-SIMD-Vector-Instructions/ABS--vector-?lang=en
I think the original code `n->as_Vector()->length() == 4 ||` is not right for basic type Byte. So I delete it, I am sorry if I miss something.

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PR: https://git.openjdk.java.net/jdk/pull/1215