[vector] Strengthen ETYPE and VLENGTH specification

[John Rose]

Nicely done; reviewed.

> On May 7, 2020, at 1:22 PM, Paul Sandoz <paul.sandoz at oracle.com> wrote:
> 
> Hi,
> 
> Please review the following change to strengthen the ETYPE and VLENGTH specification:
> 
>   http://cr.openjdk.java.net/~psandoz/panama/vector-etype-vlength-spec/webrev/ <http://cr.openjdk.java.net/~psandoz/panama/vector-etype-vlength-spec/webrev/>
> 
> This change is in response to reviews in preparation for integration into jdk/jdk.
> 
> Paul.