[llvm] [LoopVectorize] In LoopVectorize.cpp start using getSymbolicMaxBackedgeTakenCount (PR #108833)

[Florian Hahn via llvm-commits]

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@@ -4054,7 +4054,13 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
     unsigned MaxVFtimesIC =
         UserIC ? *MaxPowerOf2RuntimeVF * UserIC : *MaxPowerOf2RuntimeVF;
     ScalarEvolution *SE = PSE.getSE();
-    const SCEV *BackedgeTakenCount = PSE.getBackedgeTakenCount();
+    // Currently only loops with countable exits are vectorized, but calling
+    // getSymbolicMaxBackedgeTakenCount allows enablement work for loops with
+    // uncountable exits whilst also ensuring the symbolic maximum and known
+    // back-edge taken count remain identical for loops with countable exits.
+    const SCEV *BackedgeTakenCount = PSE.getSymbolicMaxBackedgeTakenCount();
+    assert(!isa<SCEVCouldNotCompute>(BackedgeTakenCount) &&
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fhahn wrote:

My original suggestion was along the lines of
```suggestion
    assert(BackedgeTakenCount == PSE.getBackedgeTakenCount() &&
```

https://github.com/llvm/llvm-project/pull/108833