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

[Matthias Braun via llvm-commits]

MatzeB wrote:

> Sanity check would be great to avoid unexpected downward performance change (regression) as well.

Managed to get one build from our internal "unicorn" service. Showing no regression (shows a tiny improvement that is likely benchmarking noise).

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