[llvm] [AMDGPU][SIPreEmitPeephole] mustRetainExeczBranch: use BranchProbability and TargetSchedmodel (PR #109818)

[Matt Arsenault via llvm-commits]

arsenm wrote:

We should probably skip optimizing for the unknown target, and just bail on it. We don't need to maintain a separate code path for it, it's a fake result anyway 

https://github.com/llvm/llvm-project/pull/109818