[vector] RFR 8221817: No suitable species for indexMap of Gather/Scatter VectorAPI

[Joshua Zhu (Arm Technology China)]

Hi,

JBS: https://bugs.openjdk.java.net/browse/JDK-8221817
webrev: http://cr.openjdk.java.net/~jzhu/vectorapi/8221817.indexMask/webrev.00/

I have a patch that introduces indexMask to fix no suitable species for
indexMap vectorization of Gather/Scatter VectorAPI in Long/DoubleMaxVector.
Please see JBS for details.

Besides indexMask, I also changed Long/DoubleMaxVector's gather/scatter test codes 
to avoid failure caused by IndexOutOfBoundsException for unset mask lanes [1].

This patch has been verified by testing.

[1] https://bugs.openjdk.java.net/browse/JDK-8221816

Best Regards,
Joshua