[llvm] [CodeGen] Restore MachineBlockPlacement block ordering (PR #99351)

[John Brawn via llvm-commits]

john-brawn-arm wrote:

> Collecting BlockFilterSet from the entire function will cause the complexity to increase back to O(#loops in function * #blocks)

This should be faster than before, as collectLoopBlocks is called only once per loop whereas getFirstUnplacedBlock is called for each block where it can't find a best successor, but running a quick experiment it's actually not that much faster. I'll try and see if I can come up with a better way of doing this.

https://github.com/llvm/llvm-project/pull/99351