[llvm] [BranchFolder] Prevent nested salvage in empty BB chains with pseudo probes (PR #206092)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

>  this change identifies empty BBs containing pseudo probes

How can a block contain a pseudo-probe if it's empty?

https://github.com/llvm/llvm-project/pull/206092