[vector] Add support for VectorCast nodes for AArch64

[Qi Feng]

Hi all,

Please help me review this patch. This patch adds:

1. backend support for nodes VectorCast{B,S,I,L,F,D}2X on AArch64.
2. functions to encode assembly instructions
   fcvtl/fcvtn/scvtfs/scvtfd/scvtfv/sshll/sshll2/sxtl, which are used
   needed when adding support for these nodes.

Webrev: http://cr.openjdk.java.net/~yzhang/vectorapi/vectorapi.cast/webrev.00/

Thanks,
Qi