[vector] Vector cast support

[Lupusoru, Razvan A]

Hi everyone,

I have implemented casts for almost all of the types (with limitations noted below). Since casts do not necessarily preserve the same number of elements, the implementation is actually done by using a mix of resize (previous functionality) and cast which retain same number of elements. I am happy also to report that the testing helped me find a lot of problems during the development of this - so I am glad they are so thorough.

There are two limitations right now:

-          Long casts to FP are only supported with AVX512

-          FP casts to integral types are not supported due to the complex logic needed for fixup

Let me know if you have any comments.
http://cr.openjdk.java.net/~rlupusoru/panama/webrev_casts_02/

Thanks,
Razvan