[llvm] [Coverage] Rework Decision/Expansion/Branch (PR #78969)

[Fangrui Song via llvm-commits]

MaskRay wrote:

The description is too brief for the involved problem:) Feel free to incorporate my comment https://github.com/llvm/llvm-project/issues/77871#issuecomment-1913454161 into the description/commit message.

https://github.com/llvm/llvm-project/pull/78969