[llvm] Fix SCC enter block collection in BPI (PR #210580)

[via llvm-commits]

SomeFlyingThing wrote:

> Does the problem also exist with #210301?

Yes it does fix the same issue, this Pr fixes the current implementation on main and adds a regression test. 
Do I keep this open or close to favor #210301?

https://github.com/llvm/llvm-project/pull/210580