[llvm] [VPlan] Remove unused first mask op from VPBlendRecipe. (PR #87770)

[via llvm-commits]

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@@ -1951,13 +1951,18 @@ class VPBlendRecipe : public VPSingleDefRecipe {
 
   /// Return the number of incoming values, taking into account that a single
   /// incoming value has no mask.
-  unsigned getNumIncomingValues() const { return (getNumOperands() + 1) / 2; }
+  unsigned getNumIncomingValues() const { return (getNumOperands() + 2) / 2; }
 
   /// Return incoming value number \p Idx.
-  VPValue *getIncomingValue(unsigned Idx) const { return getOperand(Idx * 2); }
+  VPValue *getIncomingValue(unsigned Idx) const {
+    return Idx == 0 ? getOperand(0) : getOperand(Idx * 2 - 1);
+  }
 
   /// Return mask number \p Idx.
-  VPValue *getMask(unsigned Idx) const { return getOperand(Idx * 2 + 1); }
+  VPValue *getMask(unsigned Idx) const {
+    assert(Idx > 0);
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ayalz wrote:

```suggestion
    assert(Idx > 0 && "First index has no mask associated");
```

https://github.com/llvm/llvm-project/pull/87770