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

To reduce conditional judges in the loop in `findIndependencePairs()`, I
have tried a couple of tweaks.

* Isolate the final result in `TestVectors`

`using TestVectors = llvm::SmallVector<std::pair<TestVector,
CondState>>;`
The final result was just piggybacked on `TestVector`, so it has been
isolated.

* Filter out and sort `ExecVectors` by the final result

It will cost more in constructing `ExecVectors`, but it can reduce at
least one conditional judgement in the loop.