[llvm] [MC/DC] Refactor: Isolate the final result out of TestVectors (PR #82282)

[NAKAMURA Takumi via llvm-commits]

chapuni wrote:

I supposed this is the draft for opinion at first time, but I think it better to land in advance of #82174 , to clarify its effect (bitcount expected equal to 1).

https://github.com/llvm/llvm-project/pull/82282