[vector] reductions with mask

[Paul Sandoz]

Hi,

Here is a patch that, for completeness, adds method for reductions with masks leveraging blend:

  http://cr.openjdk.java.net/~psandoz/panama/reduction-ops-with-mask/webrev/ <http://cr.openjdk.java.net/~psandoz/panama/reduction-ops-with-mask/webrev/>

Paul.