[vector] Fix the issue that unslice modifies the argument vector

[Yang Zhang (Arm Technology China)]

Hi all

Could anyone please help to review this patch?

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

In this patch, I fix the issue that unsliced modifies the argument vector. The reason why change the name of getElements() to vec() is described in
previous email thread.
 https://mail.openjdk.java.net/pipermail/panama-dev/2019-September/006389.html 

Regards,
Yang