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

[Phoebe Wang via llvm-commits]

phoebewang 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?

The above `MachineBlockFrequencyInfo(MachineBlockFrequencyInfo &&);`

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