[llvm] [VPlan] Don't use the legacy cost model for loop conditions (PR #156864)

[John Brawn via llvm-commits]

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@@ -1151,6 +1151,32 @@ InstructionCost VPInstruction::computeCost(ElementCount VF,
     return Ctx.TTI.getIndexedVectorInstrCostFromEnd(Instruction::ExtractElement,
                                                     VecTy, Ctx.CostKind, 0);
   }
+  case VPInstruction::BranchOnCount: {
+    // If TC <= VF then this is just a branch.
+    // FIXME: Removing the branch happens in simplifyBranchConditionForVFAndUF
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john-brawn-arm wrote:

After looking into this, the problem here is that we don't have access to the ScalarEvolution object here in VPInstruction, so putting code from simplifyBranchConditionForVFAndUF into a function and calling it won't work as that makes use of ScalarEvolution. Simplifying the branch before we compute the cost seems like a good solution to this.

https://github.com/llvm/llvm-project/pull/156864