[llvm] [RISCV][CostModel] Correct the cost of some reductions (PR #118072)

[Luke Lau via llvm-commits]

================
@@ -1470,27 +1470,27 @@ RISCVTTIImpl::getMinMaxReductionCost(Intrinsic::ID IID, VectorType *Ty,
     llvm_unreachable("Unsupported intrinsic");
   case Intrinsic::smax:
     SplitOp = RISCV::VMAX_VV;
-    Opcodes = {RISCV::VMV_S_X, RISCV::VREDMAX_VS, RISCV::VMV_X_S};
+    Opcodes = {RISCV::VREDMAX_VS, RISCV::VMV_X_S};
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lukel97 wrote:

For the max/min reductions our lowering just reuses the source vector for the start value, e.g.

```llvm
define signext i8 @vreduce_umax_nxv1i8(<vscale x 1 x i8> %v) {
; CHECK-LABEL: vreduce_umax_nxv1i8:
; CHECK:       # %bb.0:
; CHECK-NEXT:    vsetvli a0, zero, e8, mf8, ta, ma
; CHECK-NEXT:    vredmaxu.vs v8, v8, v8
; CHECK-NEXT:    vmv.x.s a0, v8
; CHECK-NEXT:    ret
  %red = call i8 @llvm.vector.reduce.umax.nxv1i8(<vscale x 1 x i8> %v)
  ret i8 %red
}
```

https://github.com/llvm/llvm-project/pull/118072