[llvm] [LLVM][VPlan] Pick more optimal initial value for VPBlend. (PR #104019)

[Paul Walker via llvm-commits]

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@@ -918,6 +918,16 @@ static void simplifyRecipe(VPRecipeBase &R, VPTypeAnalysis &TypeInfo) {
     // value with the others blended into it.
 
     unsigned StartIndex = 0;
+    for (unsigned I = 0; I != Blend->getNumIncomingValues(); ++I) {
+      // If a value's mask is only used by the blend then is can be deadcoded.
+      // TODO: Find the most expensive mask that can be deadcoded.
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paulwalker-arm wrote:

Thanks @ayalz, this was very useful.  I've added a multi-phi test to if-conversion-nest.ll to highlight the TODO.

https://github.com/llvm/llvm-project/pull/104019