[VPlan] Replace VPIRMetadata::addMetadata with setMetadata. (NFC)

Replace addMetadata with setMetadata, which sets metadata, updating
existing entries or adding a new entry otherwise.

This isn't strictly needed at the moment, but will be needed for
follow-up patches.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -970,14 +970,17 @@ class VPIRMetadata {
   /// Add all metadata to \p I.
   void applyMetadata(Instruction &I) const;
 
-  /// Add metadata with kind \p Kind and \p Node.
-  void addMetadata(unsigned Kind, MDNode *Node) {
-    assert(none_of(Metadata,
-                   [Kind](const std::pair<unsigned, MDNode *> &P) {
-                     return P.first == Kind;
-                   }) &&
-           "Kind must appear at most once in Metadata");
-    Metadata.emplace_back(Kind, Node);
+  /// Set metadata with kind \p Kind to \p Node. If metadata with \p Kind
+  /// already exists, it will be replaced. Otherwise, it will be added.
+  void setMetadata(unsigned Kind, MDNode *Node) {
+    auto It =
+        llvm::find_if(Metadata, [Kind](const std::pair<unsigned, MDNode *> &P) {
+          return P.first == Kind;
+        });
+    if (It != Metadata.end())
+      It->second = Node;
+    else
+      Metadata.emplace_back(Kind, Node);
   }
 
   /// Intersect this VPIRMetada object with \p MD, keeping only metadata

llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp

@@ -672,7 +672,7 @@ void VPlanTransforms::attachCheckBlock(VPlan &Plan, Value *Cond,
     MDBuilder MDB(Plan.getContext());
     MDNode *BranchWeights =
         MDB.createBranchWeights(CheckBypassWeights, /*IsExpected=*/false);
-    Term->addMetadata(LLVMContext::MD_prof, BranchWeights);
+    Term->setMetadata(LLVMContext::MD_prof, BranchWeights);
   }
 }
 
@@ -756,7 +756,7 @@ void VPlanTransforms::addMinimumIterationCheck(
     MDBuilder MDB(Plan.getContext());
     MDNode *BranchWeights = MDB.createBranchWeights(
         ArrayRef(MinItersBypassWeights, 2), /*IsExpected=*/false);
-    Term->addMetadata(LLVMContext::MD_prof, BranchWeights);
+    Term->setMetadata(LLVMContext::MD_prof, BranchWeights);
   }
 }
 
@@ -793,7 +793,7 @@ void VPlanTransforms::addMinimumVectorEpilogueIterationCheck(
   MDBuilder MDB(Plan.getContext());
   MDNode *BranchWeights =
       MDB.createBranchWeights(Weights, /*IsExpected=*/false);
-  Branch->addMetadata(LLVMContext::MD_prof, BranchWeights);
+  Branch->setMetadata(LLVMContext::MD_prof, BranchWeights);
 }
 
 /// If \p RedPhiR is used by a ComputeReductionResult recipe, return it.

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -4458,7 +4458,7 @@ void VPlanTransforms::addBranchWeightToMiddleTerminator(
   MDBuilder MDB(Plan.getContext());
   MDNode *BranchWeights =
       MDB.createBranchWeights({1, VectorStep - 1}, /*IsExpected=*/false);
-  MiddleTerm->addMetadata(LLVMContext::MD_prof, BranchWeights);
+  MiddleTerm->setMetadata(LLVMContext::MD_prof, BranchWeights);
 }
 
 /// Create and return a ResumePhi for \p WideIV, unless it is truncated. If the