[llvm] [SLP]Reduce number of alternate instruction, where possible (PR #128907)

[via llvm-commits]

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@@ -8612,6 +8962,47 @@ void BoUpSLP::buildTree_rec(ArrayRef<Value *> VL, unsigned Depth,
            S.getMainOp()) &&
        !all_of(VL, isVectorLikeInstWithConstOps)) ||
       NotProfitableForVectorization(VL)) {
+    if (!S) {
+      Instruction *MainOp = nullptr;
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hiraditya wrote:

Maybe we can outline this part into a separate function? looks pretty self contained.

https://github.com/llvm/llvm-project/pull/128907