Use an assert

Author: SamTebbs33

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -2840,8 +2840,8 @@ InstructionCost VPExpressionRecipe::computeCost(ElementCount VF,
                                           Ctx.CostKind);
 
   case ExpressionTypes::ExtNegatedMulAccReduction:
-    if (Opcode == Instruction::Add)
-      Opcode = Instruction::Sub;
+    assert(Opcode == Instruction::Add && "Unexpected opcode");
+    Opcode = Instruction::Sub;
     LLVM_FALLTHROUGH;
   case ExpressionTypes::ExtMulAccReduction: {
     return Ctx.TTI.getMulAccReductionCost(