[llvm] Delete assignment operator from `MachineBlockFrequencyInfo` (PR #101289)

[Shengchen Kan via llvm-commits]

KanRobert wrote:

> Since the `MachineBlockFrequencyInfo` class has an overloaded copy constructor, the assignment operator should be deleted to prevent undefined behaviour

`has an overloaded copy constructor` where, could you point out?

https://github.com/llvm/llvm-project/pull/101289