[llvm] [VPlan] Strengthen materializeFactors with assert (NFC) (PR #181665)

[Benjamin Maxwell via llvm-commits]

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@@ -5095,6 +5091,10 @@ void VPlanTransforms::materializeFactors(VPlan &Plan, VPBasicBlock *VectorPH,
   VPValue *MulByUF = Builder.createOverflowingOp(
       Instruction::Mul, {RuntimeVF, UF}, {true, false});
   VFxUF.replaceAllUsesWith(MulByUF);
+
+  assert(Plan.getVF().getNumUsers() == 0 && Plan.getUF().getNumUsers() == 0 &&
+         Plan.getVFxUF().getNumUsers() == 0 &&
+         "VF, UF, and VFxUF not expected to be used");
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MacDue wrote:

Does this capture the intention of the TODO? I interpreted as `Plan.getVF()` (etc) should not be used _again_ by another transform after `materializeFactors`. 

https://github.com/llvm/llvm-project/pull/181665