[llvm] [VPlan] Simplify BLEND %a, %b, NOT(%m) -> BLEND %b, %a, %m. (PR #128375)

[via llvm-commits]

================
@@ -851,8 +851,20 @@ static void simplifyRecipe(VPRecipeBase &R, VPTypeAnalysis &TypeInfo) {
       return;
     }
 
-    if (Blend->isNormalized())
+    if (Blend->isNormalized()) {
+      /// Simplify BLEND %a, %b, Not(%mask) -> BLEND %b, %a, %mask.
+      VPValue *NewMask;
+      if (Blend->getNumOperands() == 3 &&
+          match(Blend->getMask(1), m_Not(m_VPValue(NewMask)))) {
+        VPValue *Inc0 = Blend->getIncomingValue(0);
+        VPValue *Inc1 = Blend->getIncomingValue(1);
+        Blend->setOperand(0, Inc1);
+        Blend->setOperand(1, Inc0);
+        Blend->setOperand(2, NewMask);
----------------
ayalz wrote:

```suggestion
        Blend->setOperand(2, NewMask);
        recursivelyDeleteDeadRecipes(OldMask);
```
or check if unused and delete, knowing its operand is live? Although it does seem to get collected eventually.

https://github.com/llvm/llvm-project/pull/128375