[AArch64][GlobalISel] Implement selection for G_UNMERGE of vectors to vectors.

This re-uses the previous support for extract vector elt to extract the
subvectors.

Differential Revision: https://reviews.llvm.org/D59390

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356213 91177308-0d34-0410-b5e6-96231b3b80d8