[llvm] MachineBlockPlacement: Add tolerance to comparisons (PR #67197)

[Matthias Braun via llvm-commits]

MatzeB wrote:

FWIW: I also realized that the big majority of the instability I was tackling came from `MachineBlockPlacement::selectBestCandidateBlock`, so I decided to revert the changes to `MachineBlockPlacement::findBestLoopTopHelper` (there is still a debug print and cleanup there, but no actual change anymore).

https://github.com/llvm/llvm-project/pull/67197