RFR: 8259948: Aarch64: Add cast nodes for Aarch64 Neon backend [v4]

[Eric Liu]

On Tue, 7 Sep 2021 09:39:02 GMT, Wu Yan <wuyan at openjdk.org> wrote:

>> src/hotspot/cpu/aarch64/aarch64.ad line 2452:
>> 
>>> 2450:         return false;
>>> 2451:       }
>>> 2452:       break;
>> 
>> Why do you remove others but keep this (4Sto4B not implemented)?
>
> Because @theRealELiu suggest us not to have VectorCast with bit_size less than 64 bits support.
> In addition, we will add rules for the 4B vector, which will appear in actual situations. I'll refine the code in next commit.

Byte is special here. 

Refs:
[1] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/matcher.hpp#L341
[2] https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/aarch64/aarch64.ad#L2491

-------------

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