[msan][NFCI] Add tests for Arm NEON saturating extract narrow (#125331)

Forked from llvm/test/CodeGen/AArch64/arm64-vmovn.ll

Unknown intrinsics which are currently incorrectly handled by
visitInstruction:
- llvm.aarch64.neon.sqxtn
- llvm.aarch64.neon.sqxtun
- llvm.aarch64.neon.uqxtn

Author: thurstond