[msan][NFCI] Add tests for Arm NEON vector convert (llvm#126095)

Suboptimally handled by visitInstruction: llvm.aarch64.neon.
- fcvtas, fcvtau
- fcvtms, fcvtmu
- fcvtns, fcvtnu
- fcvtps, fcvtpu
- fcvtzs, fcvtzu
- fcvtxn
- vcvtfp2fxs, vcvtfp2fxu
- vcvtfxs2fp, vcvtfxu2fp

Forked from llvm/test/CodeGen/AArch64/arm64-{cvt,vcvt}.ll

Author: thurstond