[llvm] [VPlan] Run initial recipe simplification on VPlan0. (PR #176828)

[Ramkumar Ramachandra via llvm-commits]

================
@@ -1219,15 +1219,25 @@ static void simplifyRecipe(VPSingleDefRecipe *Def, VPTypeAnalysis &TypeInfo) {
   }
 
   VPBuilder Builder(Def);
+
+  // Avoid replacing VPInstructions with underlying values with new
+  // VPInstructions. VPInstructions with underlying values should get
+  // widened/replicated later.
+  // TODO: We should also not replace non-VPInstructions like VPWidenRecipe with
+  // VPInstructions without underlying values, as those will get skipped during
+  // cost computation.
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artagnon wrote:

```suggestion
  // Avoid replacing VPInstructions with underlying values with new
  // VPInstructions, as we would fail to create Widen-recipes from the new VPInstructions without an underlying value, and miss out on some Widen-only transformations later, by doing so.
  // TODO: We should also not replace non-VPInstructions like VPWidenRecipe with
  // VPInstructions without underlying values, as those will get skipped during
  // cost computation.
```

I had a question: why can't we just set the underlying value of the newly created VPInstruction? What would be the effect of that?

https://github.com/llvm/llvm-project/pull/176828