[Attributor][NFC] Do not manifest noundef for positions to be changed to undef

[okura]

Summary

This patch fixes AANoUndef manifestation.
We should not manifest noundef for positions that will be changed to undef.

[jdoerfert]

LGTM Thx

[jdoerfert]

I think this is fine even though the reason it was created might not need it after all.

llvm/lib/Transforms/IPO/AttributorAttributes.cpp

struct AANoUndefImpl : AANoUndef {
   }
 
   ChangeStatus manifest(Attributor &A) override {
     // We don't manifest noundef attribute for dead positions because the
     // associated values with dead positions would be replaced with undef
     // values.
     if (A.isAssumedDead(getIRPosition(), nullptr, nullptr))
       return ChangeStatus::UNCHANGED;
+    // A position whose simplified value does not have any value is
+    // considered to be dead. We don't manifest noundef in such positions for
+    // the same reason above.
+    auto &ValueSimplifyAA = A.getAAFor<AAValueSimplify>(
+        *this, getIRPosition(), /* TrackDependence */ false);
+    if (!ValueSimplifyAA.getAssumedSimplifiedValue(A).hasValue())
+      return ChangeStatus::UNCHANGED;
     return AANoUndef::manifest(A);
   }
 };
 
 struct AANoUndefFloating : public AANoUndefImpl {
   AANoUndefFloating(const IRPosition &IRP, Attributor &A)
       : AANoUndefImpl(IRP, A) {}