[llvm] [VPlan] Move initial skeleton construction earlier (NFC). (PR #150848)

[Ramkumar Ramachandra via llvm-commits]

================
@@ -488,12 +488,46 @@ void VPlanTransforms::prepareForVectorization(
 
   addCanonicalIVRecipes(Plan, HeaderVPBB, LatchVPBB, InductionTy, IVDL);
 
-  [[maybe_unused]] bool HandledUncountableEarlyExit = false;
+  // Create SCEV and VPValue for the trip count.
+  // We use the symbolic max backedge-taken-count, which works also when
+  // vectorizing loops with uncountable early exits.
+  const SCEV *BackedgeTakenCountSCEV = PSE.getSymbolicMaxBackedgeTakenCount();
+  assert(!isa<SCEVCouldNotCompute>(BackedgeTakenCountSCEV) &&
+         "Invalid loop count");
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artagnon wrote:

```suggestion
         "Invalid backedge-taken count");
```

Are we guaranteed not to hit this assert?

https://github.com/llvm/llvm-project/pull/150848