[llvm] [LV][EVL] Skip tryAddExplicitVectorLength for plans with scalar VF. (PR #125497)

[David Sherwood via llvm-commits]

================
@@ -8925,7 +8925,7 @@ void LoopVectorizationPlanner::buildVPlansWithVPRecipes(ElementCount MinVF,
       VPlanTransforms::optimize(*Plan);
       // TODO: try to put it close to addActiveLaneMask().
       // Discard the plan if it is not EVL-compatible
-      if (CM.foldTailWithEVL() &&
+      if (CM.foldTailWithEVL() && !Plan->hasScalarVF() &&
----------------
david-arm wrote:

I think perhaps you don't need the new `hasScalarVF` function because it looks like we already do something similar above on line 8922, i.e. you could do this instead:

```
  if (auto Plan = tryToBuildVPlanWithVPRecipes(SubRange)) {
    // Now optimize the initial VPlan.
    bool HasScalarVF = Plan->hasVF(ElementCount::getFixed(1));
    if (!HasScalarVF)
        VPlanTransforms::runPass(VPlanTransforms::truncateToMinimalBitwidths,
                                 *Plan, CM.getMinimalBitwidths());
...
    if (CM.foldTailWithEVL() && !HasScalarVF &&
...
```

https://github.com/llvm/llvm-project/pull/125497