[llvm] [VPlan] Simplify before convertToConcreteRecipes, remove SCALAR-STEPS. (PR #123655)

[David Sherwood via llvm-commits]

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@@ -890,6 +890,17 @@ static void simplifyRecipe(VPRecipeBase &R, VPTypeAnalysis &TypeInfo) {
     return;
   }
 
+  // VPScalarIVSteps can only be simplified after unrolling. VPScalarIVSteps for
+  // part 0 can be replaced by their start value, if only the first lane is
+  // demanded.
+  if (auto *Steps = dyn_cast<VPScalarIVStepsRecipe>(&R)) {
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david-arm wrote:

Hi @fhahn, what I meant in one of my earlier comments was about taking everything in this patch *except* this new `VPScalarIVStepsRecipe` code. Then using that as a standalone PR, just to make it easier to see which tests are affected by this code and which are affected by the changes to `executePlan`. I tested this out downstream by building everything in this patch *except* this code and found some tests crash at Line 962 for this recipe `WIDEN ir<%mul16> = vp.mul vp<%3>, vp<%10>, vp<%7>` and the assert is:

```
bool llvm::VPlanPatternMatch::Recipe_match<Ops_t, Opcode, Commutative, RecipeTys>::match(const llvm::VPRecipeBase*) const [with Ops_t = std::tuple<llvm::VPlanPatternMatch::bind_ty<llvm::VPValue>, llvm::VPlanPatternMatch::specific_intval<0> >; unsigned int Opcode = 17; bool Commutative = true; RecipeTys = {llvm::VPWidenRecipe, llvm::VPReplicateRecipe, llvm::VPWidenCastRecipe, llvm::VPInstruction}]: Assertion `R->getNumOperands() == std::tuple_size<Ops_t>::value && "recipe with matched opcode the expected number of operands"' failed.
```

https://github.com/llvm/llvm-project/pull/123655