[GlobalISel][AArch64] Add partial selection support for G_INSERT_VECTOR_ELT

This adds support for inserting elements into packed vectors. It also adds
two tests: one for selection, and one for regbank select.

Unpacked vectors will come in a follow-up.

Differential Revision: https://reviews.llvm.org/D59325

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356182 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed