[msan][NFCI] Add tests for NEON saturating extract and narrow (#125140)

Copied from llvm/test/CodeGen/AArch64/qshrn.ll

Unknown instructions (not correctly handled):
- llvm.aarch64.neon.sqxtn.v2i32, llvm.aarch64.neon.sqxtn.v4i16,
llvm.aarch64.neon.sqxtn.v8i8
- llvm.aarch64.neon.sqxtun.v2i32, llvm.aarch64.neon.sqxtun.v4i16,
llvm.aarch64.neon.sqxtun.v8i8
- llvm.aarch64.neon.uqxtn.v2i32, llvm.aarch64.neon.uqxtn.v4i16,
llvm.aarch64.neon.uqxtn.v8i8

Author: thurstond