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

[Tim Creech via llvm-commits]

tcreech-intel wrote:

!unpredictable metadata is preserved as a MachineInstr flag and may be useful to backends too, such as here:
https://github.com/llvm/llvm-project/blob/1a80153ba91f1e623c042fa0ae1ee5ab67087c0e/llvm/lib/Target/X86/X86CmovConversion.cpp#L310-L314

Also I see an additional use of IR !unpredictable is proposed in #98495. (Ping @HaohaiWen, @williamweixiao, @tianqingw for awareness/feedback.)


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