[llvm] [VPlan] Simplify branch-cond with getVectorTripCount (PR #155604)

[Luke Lau via llvm-commits]

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@@ -1483,14 +1483,15 @@ static bool simplifyBranchConditionForVFAndUF(VPlan &Plan, ElementCount BestVF,
     // Try to simplify the branch condition if TC <= VF * UF when the latch
     // terminator is   BranchOnCount or BranchOnCond where the input is
     // Not(ActiveLaneMask).
-    const SCEV *TripCount =
-        vputils::getSCEVExprForVPValue(Plan.getTripCount(), SE);
-    assert(!isa<SCEVCouldNotCompute>(TripCount) &&
+    const SCEV *VectorTripCount =
+        vputils::getSCEVExprForVPValue(&Plan.getVectorTripCount(), SE);
+    if (isa<SCEVCouldNotCompute>(VectorTripCount))
+      VectorTripCount = vputils::getSCEVExprForVPValue(Plan.getTripCount(), SE);
+    assert(!isa<SCEVCouldNotCompute>(VectorTripCount) &&
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lukel97 wrote:

> We don't have a vector trip count when there is a scalar tail.

Should that be the other way round, i.e we don't have a vector trip count when there is no scalar tail? The tests in this PR that are affected seem to have scalar tails

https://github.com/llvm/llvm-project/pull/155604