[llvm] [RISCV] Use legally typed splat during vmv_v_v splat(x) -> vmv_v_x (PR #173154)

[Craig Topper via llvm-commits]

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@@ -21974,7 +21974,7 @@ SDValue RISCVTargetLowering::PerformDAGCombine(SDNode *N,
     if (sd_match(Src, m_InsertSubvector(m_Undef(), m_Value(SubVec), m_Zero())))
       Src = SubVec;
 
-    SDValue SplatVal = DAG.getSplatValue(Src);
+    SDValue SplatVal = DAG.getSplatValue(Src, true);
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topperc wrote:

```suggestion
    SDValue SplatVal = DAG.getSplatValue(Src, /*LegalTypes=*/true);
```

https://github.com/llvm/llvm-project/pull/173154