[llvm] [profcheck] Add unknown branch weights to expanded cmpxchg loop. (PR #165841)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

Not sure I understand this; if you're going to treat it as unknown 50/50, wouldn't that be the default without adding the metadata? In this context a 50/50 is a poor guess, since 36cb33bbcae8a7f988e101f5c9b23e7c68f73808 we set likely branch weights 

https://github.com/llvm/llvm-project/pull/165841