[vector] Clarify the equivalent behavior of Vector.reduceLanesToLong

[John Rose]

Good clarification.

> On May 11, 2020, at 5:20 PM, Paul Sandoz <paul.sandoz at oracle.com> wrote:
> 
> Hi,
> 
> Please review the following change to clarify the equivalent behavior of Vector.reduceLanesToLong:
> 
>  http://cr.openjdk.java.net/~psandoz/panama/vector-reduceLanesToLong-equiv-spec/webrev/ <http://cr.openjdk.java.net/~psandoz/panama/vector-reduceLanesToLong-equiv-spec/webrev/>
> 
> This change is in response to reviews in preparation for integration into jdk/jdk.
> 
> Paul.