[Attributor] Fix AANoUndef initialization

[okura]

Summary

When the associated value is undef, we immediately forced to indicate a pessimistic fixpoint so far.
This patch changes the initialization to check the attribute given in IR at first and to indicate an optimistic fixpoint when it is given.
This change will enable us to catch , for example, the following case in AAUB.

call void @foo(i32 noundef undef)

[jdoerfert]

llvm/lib/Transforms/IPO/AttributorAttributes.cpp

7797

can we move the AANoUndef::initialize(A); unconditionally to the beginning of this function?

[okura]

llvm/lib/Transforms/IPO/AttributorAttributes.cpp

7797

I think we cannot because IRAttribute::initialize indicates an optimistic fixpoint when the associated value is undef.

[jdoerfert]

LGTM.

llvm/lib/Transforms/IPO/AttributorAttributes.cpp

 };
 
 /// ------------------------ NoUndef Attribute ---------------------------------
 struct AANoUndefImpl : AANoUndef {
   AANoUndefImpl(const IRPosition &IRP, Attributor &A) : AANoUndef(IRP, A) {}
 
   /// See AbstractAttribute::initialize(...).
   void initialize(Attributor &A) override {
+    if (getIRPosition().hasAttr({Attribute::NoUndef})) {
+      indicateOptimisticFixpoint();
+      return;
+    }
     Value &V = getAssociatedValue();
     if (isa<UndefValue>(V))
       indicatePessimisticFixpoint();
     else if (isa<FreezeInst>(V))
       indicateOptimisticFixpoint();
     else if (getPositionKind() != IRPosition::IRP_RETURNED &&
              isGuaranteedNotToBeUndefOrPoison(&V))
       indicateOptimisticFixpoint();
     else
       AANoUndef::initialize(A);
   }
 
   /// See followUsesInMBEC
   bool followUseInMBEC(Attributor &A, const Use *U, const Instruction *I,
                        AANoUndef::StateType &State) {
     const Value *UseV = U->get();
     const DominatorTree *DT = nullptr;
     if (Function *F = getAnchorScope())