[vector] Clarify the equivalent behavior of Vector.reduceLanesToLong

[Paul Sandoz]

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.