[llvm] [WPD]: Enable speculative devirtualizatoin. (PR #159048)

[Hassnaa Hamdi via llvm-commits]

hassnaaHamdi wrote:

> > @teresajohnson I think it's better to handle the object visibility in this patch too ? Direct devirtualization for hidden objects and speculative for non-hidden ones ?
> 
> I could go either way. It is fine if you want to add that here, but I'm also fine with doing it as a follow up which might make the patches a bit more compact (unless you think adding the support will cause many changes to what is added by this PR).

I think the change of handling the visibility will just pass the object visibility to 'applySingleImplDevirt' to check if we apply speculative/direct devirtualization. So the changes will include changing the function prototype, and passing the parameter.
something like:

```
void DevirtModule::applySingleImplDevirt(VTableSlotInfo &SlotInfo,
                                         Constant *TheFn, bool &IsExported,
                                         std::optional<GlobalObject::VCallVisibility> ObjVisibility) {
 ..  
 if (DevirtCheckMode == WPDCheckMode::Fallback || (speculativeDevirt && ObjVisibility != hidden)
      ... apply speculative devirt.
  else // it will do the direct devirt.
}

bool DevirtModule::trySingleImplDevirt(
    ModuleSummaryIndex *ExportSummary,
    MutableArrayRef<VirtualCallTarget> TargetsForSlot, VTableSlotInfo &SlotInfo,
    WholeProgramDevirtResolution *Res) {
 ...
 applySingleImplDevirt(SlotInfo, TheFn, IsExported, TargetsForSlot.TM.Bits->GV->getVCallVisibility());
} 
```

https://github.com/llvm/llvm-project/pull/159048