[llvm] [nfc][instr] Encapsulate CFGMST (PR #72207)

[Mircea Trofin via llvm-commits]

mtrofin wrote:

I'm pretty sure the `public` right under the `class` declaration was meant to be `private` - there was another `public` right above the ctor.

https://github.com/llvm/llvm-project/pull/72207