[vector] RFR: Rematerialization support for vector masks

[Vladimir Ivanov]

http://cr.openjdk.java.net/~vlivanov/panama/vector/mask_rematerialization/webrev.00/

Vector masks need special treatment in JVM to support rematerialization 
at safepoints.

Also, temporarily converted Vector.Mask and Vector.Shuffle from 
interfaces to abstract classes to overcome limitations of vector box 
elimination implementation. Once the analysis is enhanced to cover 
interfaces, they should be converted back.

Contributed-by: vlivanov, rlupusoru

Thanks!

Best regards,
Vladimir Ivanov