[llvm] [LV] Stop using the legacy cost model for udiv + friends (PR #152707)

[David Sherwood via llvm-commits]

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@@ -952,6 +952,19 @@ InstructionCost VPInstruction::computeCost(ElementCount VF,
   }
 
   switch (getOpcode()) {
+  case Instruction::Select: {
+    // TODO: It may be possible to improve this by analyzing where the
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david-arm wrote:

Yep, I think that's why I had to update `selectVectorizationFactor` to add the select cost to the legacy model too. By making changes to remove the legacy cost model from vplan we end up triggering the legacy/vplan cost model assert, which means we then have to do extra work to fix the assert making the patch bigger than it should be.

https://github.com/llvm/llvm-project/pull/152707