[llvm] [VPlan] Materialize constant vector trip counts before final opts. (PR #142309)

[via llvm-commits]

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@@ -3120,6 +3115,29 @@ void VPlanTransforms::materializeBroadcasts(VPlan &Plan) {
   }
 }
 
+void VPlanTransforms::materializeVectorTripCount(
+    VPlan &Plan, ElementCount BestVF, unsigned BestUF,
+    PredicatedScalarEvolution &PSE) {
+  assert(Plan.hasVF(BestVF) && "BestVF is not available in Plan");
+  assert(Plan.hasUF(BestUF) && "BestUF is not available in Plan");
+
+  VPValue *TC = Plan.getTripCount();
+  // Skip cases for which the trip count may be non-trivial to materialize.
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ayalz wrote:

```suggestion
  // Skip cases for which the trip count may be non-trivial to materialize.
  // I.e., when a scalar tail is absent - due to tail folding, or when a scalar tail is required.
```
An absent scalar tail due to knowing that VecTC is a multiple of VF*UF, requires materializing VecTC, so can be ignored here?

https://github.com/llvm/llvm-project/pull/142309