[llvm] [AMDGPU][StructurizeCFG] Maintain branch MD_prof metadata (PR #109813)

[Matt Arsenault via llvm-commits]

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@@ -578,12 +621,14 @@ void StructurizeCFG::insertConditions(bool Loops) {
     Dominator.addBlock(Parent);
 
     Value *ParentValue = nullptr;
-    for (std::pair<BasicBlock *, Value *> BBAndPred : Preds) {
+    MaybeCondBranchWeights ParentWeights = std::nullopt;
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arsenm wrote:

```suggestion
    MaybeCondBranchWeights ParentWeights;
```

https://github.com/llvm/llvm-project/pull/109813