[llvm] [llvm][GlobalOpt] Optimize statically resolvable IFuncs (PR #80606)

[Jon Roelofs via llvm-commits]

================
@@ -2404,6 +2405,42 @@ static bool OptimizeEmptyGlobalCXXDtors(Function *CXAAtExitFn) {
   return Changed;
 }
 
+static Function *hasSideeffectFreeStaticResolution(GlobalIFunc &IF) {
+  Function *Resolver = IF.getResolverFunction();
+  if (!Resolver)
+    return nullptr;
+
+  Function *Callee = nullptr;
+  for (BasicBlock &BB : *Resolver) {
+    if (any_of(BB, [](Instruction &I) { return I.mayHaveSideEffects(); }))
+      return nullptr;
+
+    if (auto *Ret = dyn_cast<ReturnInst>(BB.getTerminator()))
+      if (auto *F = dyn_cast<Function>(Ret->getReturnValue())) {
----------------
jroelofs wrote:

Verifier has:
```
  Check(isa<PointerType>(Resolver->getFunctionType()->getReturnType()),
        "IFunc resolver must return a pointer", &GI);
```

https://github.com/llvm/llvm-project/pull/80606