[llvm] [SLP]Use getExtendedReduction cost and fix reduction cost calculations (PR #117350)

[Philip Reames via llvm-commits]

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@@ -2765,6 +2765,18 @@ class BasicTTIImplBase : public TargetTransformInfoImplCRTPBase<T> {
                                            Type *ResTy, VectorType *Ty,
                                            FastMathFlags FMF,
                                            TTI::TargetCostKind CostKind) {
+    if (auto *FTy = dyn_cast<FixedVectorType>(Ty);
+        FTy && Opcode == Instruction::Add &&
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preames wrote:

You're missing the IsUnsigned check here - this allows sext too.  

https://github.com/llvm/llvm-project/pull/117350