[vector] Address review comments for AArch64 backend changes

[Yang Zhang]

Hi,

Please help to review this patch that addresses review comments for Vector API AArch64 backend changes [1].
1. Seperate newly added NEON instructions to a new ad file
   aarch64_neon.ad
2. Add assembler tests for NEON instructions. Trailing spaces
   in the python script are also removed.

Webrev: http://cr.openjdk.java.net/~yzhang/vectorapi/vectorapi.neon_ad/webrev.00/

[1] https://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2020-May/038313.html

Regards
Yang