[LV] Add missing check, drop 'then'.

Address post-commit comments for 67f4968.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7003,10 +7003,11 @@ LoopVectorizationCostModel::getInstructionCost(Instruction *I,
         VectorTy->isVectorTy() ? ToVectorTy(SrcScalarTy, VF) : SrcScalarTy;
 
     if (canTruncateToMinimalBitwidth(I, VF)) {
-      // If the result type is <= then the source type, there will be no extend
+      // If the result type is <= the source type, there will be no extend
       // after truncating the users to the minimal required bitwidth.
       if (VectorTy->getScalarSizeInBits() <= SrcVecTy->getScalarSizeInBits() &&
-          (I->getOpcode() == Instruction::ZExt || I->getOpcode()))
+          (I->getOpcode() == Instruction::ZExt ||
+           I->getOpcode() == Instruction::SExt))
         return 0;
     }