[Attributor] Do not add AA to dependency graph after the update stage

[okura]

Summary

If an AA is registered to the dependency graph in the manifest stage, Attributor aborts in ::manifestAttributes().
This patch prevents such termination.

[jdoerfert]

LGTM

[okura]

Another way to achieve the same goal is to remove the assertion in manifestAttributes.
I'm not sure which is better.

[okura]

This is accepted, I'll go with this.

llvm/include/llvm/Transforms/IPO/Attributor.h

template <typename AAType> AAType &registerAA(AAType &AA) {
     // Put the attribute in the lookup map structure and the container we use to
     // keep track of all attributes.
     const IRPosition &IRP = AA.getIRPosition();
     AbstractAttribute *&AAPtr = AAMap[{&AAType::ID, IRP}];
 
     assert(!AAPtr && "Attribute already in map!");
     AAPtr = &AA;
 
+    // Register AA with the synthetic root only before the manifest stage.
+    if (Phase == AttributorPhase::SEEDING || Phase == AttributorPhase::UPDATE)
       DG.SyntheticRoot.Deps.push_back(
           AADepGraphNode::DepTy(&AA, unsigned(DepClassTy::REQUIRED)));
 
     return AA;
   }
 
   /// Return the internal information cache.
   InformationCache &getInfoCache() { return InfoCache; }
 
   /// Return true if this is a module pass, false otherwise.