[llvm] Refactor processing of BranchRegions associated with an MCDCDecisionRegion (PR #78819)

[NAKAMURA Takumi via llvm-commits]

chapuni wrote:

@evodius96 This also works for me, with #78966.
One of my issues was seen in the middle of my debugging. It produced like;
```
Decision
Decision
Branch [1,0,2]
Branch [1,0,2]
Branch [2,0,0]
Branch [2,0,0]
```
I cannot reproduce any more. AFAIK each group of branch is clustering and your impl can handle them as well.

This is shorter however my impl is more descriptive (I think). Both are working to me. Which could we take?

https://github.com/llvm/llvm-project/pull/78819