[llvm] Improvements to RS4GC BDV Algorithm (PR #69795)

[via llvm-commits]

https://github.com/annamthomas approved this pull request.

Change looks good to go (just a nit inline). Can you pls squash all commits together?
Will wait for a day or so to see if other reviewers have any comments (since we already discussed this problem and solution at length :) ). 
Also, Petr do you have access to commit upstream?

https://github.com/llvm/llvm-project/pull/69795