[llvm] [RISCV][CostModel] Estimate cost of llvm.vector.reduce.fmaximum/fminimum (PR #80697)

[Philip Reames via llvm-commits]

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@@ -1001,6 +1001,53 @@ RISCVTTIImpl::getMinMaxReductionCost(Intrinsic::ID IID, VectorType *Ty,
       return getArithmeticReductionCost(Instruction::And, Ty, FMF, CostKind);
   }
 
+  if (IID == Intrinsic::maximum || IID == Intrinsic::minimum) {
+    SmallVector<unsigned, 5> SplitOps;
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preames wrote:

Can you subset this patch to the case where LT.first == 1?  The splitting logic still looks too conservative here, but let's separate that discussion off.  

https://github.com/llvm/llvm-project/pull/80697