[llvm] [Codegen] (NFC) Faster algorithm for MachineBlockPlacement (PR #91843)

[William Junda Huang via llvm-commits]

huangjd wrote:

> fwiw, https://llvm-compile-time-tracker.com/ is often used to measure compile-time changes for upstream changes. It has a ThinLTO set-up but there isn't one for PGO. https://llvm-compile-time-tracker.com/about.php has the instructions if you are interested.

Let me try set that up, although this patch is quite specifically dealing with code containing many loops in a sequence (for example protobuf generated file), not sure if the benchmark can show a significant difference. 

https://github.com/llvm/llvm-project/pull/91843