CodeGen: s/protected:/private:/ in MachineBasicBlock, NFC

Nothing inherits from `MachineBasicBlock`, so this should have no real
functionality change.  Just makes the code easier to understand.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249473 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed