[vector] RFR fix masked gather load and add masked gather/scatter tests

[Paul Sandoz]

Hi,

Please review a fix to the masked gather load and add back relevant tests:

  http://cr.openjdk.java.net/~psandoz/panama/vector-scatter-gather-mask/webrev/ <http://cr.openjdk.java.net/~psandoz/panama/vector-scatter-gather-mask/webrev/>

For the now it is easier to take the slow path when the mask bits are not all set.  This needs to be revisited, as in general to do all masked load/stores.

Paul.