[msan] Add more Arm NEON vector add tests

Handled heuristically but incorrectly:
- llvm.aarch64.neon.{addp, faddp}

Unknown instructions handled by visitInstruction:
- llvm.aarch64.neon.{addhn, raddhn}
- llvm.aarch64.neon.{saddlp, uaddlp}

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

Author: thurstond