[llvm] [VPlan] Simplify VPBlendRecipes to select instructions (PR #133993)

[Luke Lau via llvm-commits]

================
@@ -928,6 +928,19 @@ InstructionCost VPInstruction::computeCost(ElementCount VF,
     return Ctx.TTI.getVectorInstrCost(Instruction::ExtractElement, VecTy,
                                       Ctx.CostKind);
   }
+  case Instruction::Select: {
+    if (!getUnderlyingValue())
----------------
lukel97 wrote:

Is it possible for a VPInstruction to be a live in? A live in has to not have a recipe:

```c++
  /// Returns true if this VPValue is a live-in, i.e. defined outside the VPlan.
  bool isLiveIn() const { return !hasDefiningRecipe(); }
```

https://github.com/llvm/llvm-project/pull/133993