[AArch64][GlobalISel] Add isel support for a couple vector exts/truncs

Add support for

- v4s16 <-> v4s32
- v2s64 <-> v2s32

And update tests that use them to show that we generate the correct
instructions.

Differential Revision: https://reviews.llvm.org/D57832

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353732 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed