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

[Mircea Trofin via llvm-commits]

mtrofin wrote:

> 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 [36cb33b](https://github.com/llvm/llvm-project/commit/36cb33bbcae8a7f988e101f5c9b23e7c68f73808) we set likely branch weights

see https://discourse.llvm.org/t/rfc-profile-information-propagation-unittesting/73595


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