[AArch64][GlobalISel] Add some support for G_CONCAT_VECTORS.

Handles concatenating 2 x v2s32 and 2 x v4s16

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356212 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed