[llvm] MachineBlockPlacement: Avoid overflow problems in scaleThreshold() (PR #68272)

[Matthias Braun via llvm-commits]

MatzeB wrote:

Note that creating a robust unit test is probably not feasible. I could abuse the current state of BFI where large values and overflows are common, but when I land #66285 a test for this would require >= 1024 instruction to trigger the problem. So I hope this is good without one.

https://github.com/llvm/llvm-project/pull/68272