[llvm] [SLP][NFC] Extract VectorizableTree into own ForestTree class (PR #222805)

[Ryan Buchner via llvm-commits]

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@@ -8215,28 +8354,28 @@ BoUpSLP::findExternalStoreUsersReorderIndices(TreeEntry *TE) const {
   return ExternalReorderIndices;
 }
 
-void BoUpSLP::buildTree(ArrayRef<Value *> Roots,
+void BoUpSLP::buildTree(ForestTree &FT, ArrayRef<Value *> Roots,
                         const SmallDenseSet<Value *> &UserIgnoreLst) {
   deleteTree();
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bababuck wrote:

I dropped the clear because the flow changed to creating `ForestTree` on the stack each time before calling `buildTree`. Would you prefer to stick with trying to keep the `ForestTree` alive for longer and clearing it each attempt?

https://github.com/llvm/llvm-project/pull/222805