[llvm] [VPlan] Don't apply predication discount to non-originally-predicated blocks (PR #160449)

[Luke Lau via llvm-commits]

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@@ -5088,7 +5108,7 @@ InstructionCost LoopVectorizationCostModel::expectedCost(ElementCount VF) {
     // cost by the probability of executing it. blockNeedsPredication from
     // Legal is used so as to not include all blocks in tail folded loops.
     if (VF.isScalar() && Legal->blockNeedsPredication(BB))
-      BlockCost /= getPredBlockCostDivisor(CostKind);
+      BlockCost /= getPredBlockCostDivisor(CostKind, BB);
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lukel97 wrote:

I've removed the redundant `Legal->blockNeedsPredication(BB)` in cf6b43532792f46850c158c54c9dc28c912b7148 and updated the comment. I figured we're going to need to plumb through the basic block anyway in #158690 so we might as well do it in this PR

https://github.com/llvm/llvm-project/pull/160449