[llvm] Introduce UnpredictableProfileLoader for PMU branch-miss profiles (PR #99027)

[via llvm-commits]

WenleiHe wrote:

Before we get into patch review, could you start an RFC to outline: 1) high level design (also including how this interacts with other PGO), 2) performance results, 3) future plan? 

While I understand the rational behind this optimization, I think it'd be useful to show some results on benchmarks (e.g. on spec, how much additional perf we're getting here on top of sample PGO) before we introduce this in LLVM upstream. 

https://github.com/llvm/llvm-project/pull/99027